I'm going to see if I can clump all the responses from the newer divers here together and discuss them collectively.
* What would you do if you started feeling really cold on a dive...so cold that it was difficult to think clearly?
See if my first scuba instructor was around. He was hot 
OK, seriously I'd thumb the dive and get to where it's warm. I'd also signal my buddy that my brain is loopy so he'll pay close attention to me as we surface.
Thumb the dive (assuming I'm level headed enough to do so).
Signal my buddy there was a problem, I'm cold, and I'm struggling to think (Crazy person finger wobble at the temple?) Then ascend and try and get warm.
I'd have been on the way up before that. I've been cold enough in my life already, thank you.
I would really hope I had enough thinking ability to know that it is time to thumb the dive and get warm. I have been midly chilly before, but I didn't feel like I needed to end the dive.
I'm not real sure that I would get sooooo cold that I could not think clearly, I would call the dive before that point, that said, I did a dive in december where I broke the seal on a dry glove on entry and that hand was pretty cold throughout the dive. By the time my buddy called the dive 30 or 35 min later, there was certainly no argument from me
Time to thumb the dive and ascend. I would think that any situation that makes clear thinking would be a good time to end the dive, or at least ascend to where you can think clearly like in a case of narcosis.
I would signal my buddy as soon I started feeling cold. If I feel ok to keep going, I would, but I would continue to let my buddy know if I got colder. I wouldn’t let myself get so cold to the point of not thinking, but if it happened, I would thumb the dive and seek a warm blanket so I don't end up with hypothermia.
You all gave the "correct" answer in terms of thumbing the dive. Indeed, indicate to your buddy that you're cold ... and if it's starting to make you question your thinking capability, give the "I'm loopy" signal so that your buddy knows it's a serious issue.
If I'm at that point, I'm also going to tell my buddy to "watch me" ... (point toward your mask, then toward your chest) ... and possibly even maintain touch contact on the ascent.
As most of you indicated, it's a great idea to not let it get to that point. Once you begin to feel cold, let your dive buddy know. Sometimes, increasing your activity (i.e. swimming around) can help. If that's not an option, or if it doesn't help, take steps to end the dive immediately. This is NOT a time to start worrying about ruining your buddy's good time ... you'll do that a lot more effectively if you allow yourself to get to the point where they have to help you out of the water.
And that brings up another issue ... don't think that if it's just your hands or toes getting cold that you'll be OK ... you still have to manipulate your equipment, and either get up a boat ladder or walk out of the water on shore. You need functioning hands and feet to do that. If you start losing feeling in fingertips or toes, it's time to end the dive.
* What would you do if you found yourself breathing really fast/hard underwater and the reg felt like it wasn't giving you enough gas?
I'm not generally a hard/fast breather, at least I haven't been up to this point so something must have freaked me out. My first thought would be to surface and stop freaking out. However, if it was a case of not freaking out but just having a hard time getting air (but still getting some) then I'd check to make sure I actually had enough air by looking at the (hopefully functional) gauge. If not then I signal low air to my buddy, get his alternate and we surface. If I do have enough air, then maybe my valve isn't turned on all the way, or the reg has a problem. I'd have my buddy check the valve and I'd try my alternate to see if it's any better. At any rate, surfacing sooner rather than later would be in order. Funny how a lot of my answers to these questions involve getting to the surface. 
I think I remember this from class! Calm down, stop moving, and focus on good breathing until feeling goes away, otherwise thumb dive.
I have had this and it was scary, I could feel the panic rising. I signalled to stop, clung to a rock (we were finning against current) then focus on slow deep breaths. It took a good few minutes to calm down. Was able to continue after that.
I'd remember that overbreathing alone might be the problem, but it's all the same problem in the end. I'd do my self body check for physical stress responses and alter them (stress is physical, and you can dissipate most of it by correcting body responses), and that includes restoring proper breathing if I'd somehow not been doing it. If slowing the rate didn't relieve the reg problem, I'd try the backup while starting up and be doing all this fast enough to preserve what reserve I had in my lungs. The body response thing is pretty automatic with me, and once noticed, I'll deal with it automatically and can attend to the other stuff, like lining up the backup.
I believe I remember in OW class being told to stop where I was and catch my breath. I don't know if it ever happens that this feeling continues even after resting, but if so, it is time to end the dive.
First, check gauges (should be doing that anyways right ) if low on air, signal buddy, obtain air source, end dive. If there is sufficient air, signal buddy. Slow breathing (deep slow breaths) check, or have buddy check to ensure tank valve is completly open, if it is, obtain buddy's octo, and end dive to determine the source of the problem on the surface.
I think I would alert my buddy to the fact that I was having trouble and try to calm down and see if that helps. If not, I would signal to begin an ascent but be prepared to make it a shared air ascent if it were to become necessary. At a minimum I would want my buddy to be as close as comfortably possible at the time should I need his assistance.
Stop, signal buddy, calm down, breath slowly, catch my breath and maintain breath control. Continue the dive once I catch my breath and if I feel comfortable with continuing.
This could be either an equipment issue, or an exertion issue. If you're working hard, suspect the latter ... CO2 is building up in your body and you need to get it under control. Stop ... if possible, grab ahold of something and totally relax your body. Slow your breathing down ... deep and slow, to allow your lungs to flush the CO2 out of your body. After a few breaths you'll start to feel better. Then deal with whatever caused the problem in the first place. If it's because you're fighting current, for example, it might be a good time to consider that conditions aren't right for continuing the dive, and start making your way back to your exit.
If exertion isn't the problem, then you're working to hard too breathe. This is usually due to one of two things ... either you've got your second stage turned all the way down (on adjustable second stages), or your tank valve's just barely on. Signal your dive buddy you've got a problem, and check both. If both seem to be OK, switch to your alternate reg and thumb the dive. Don't continue the dive if the alternate seems to be OK ... because you don't know what's causing the issue with the reg, and it would be better to not use that reg again until you find out.
* What would you do if you're finning hard to keep your head above the surface of the water...and you're getting tired?
Is my BC broken, why is there no air in it? The first thing I do on the surface is put air in my BC. I'd ditch my weights if my BC wasn't keeping me up.
Inflate BC, lay on my back, if absolutely necessary ditch weight.
Assuming I'm OOA and can't power inflate my BCD, I'd try and manually inflate. If I couldn't do that I'd signal the boat I was in trouble and need help then ditch weight. Try and roll onto my back, ask my buddy for help.
Tell myself, "Whoa, you know better than that and know that you float okay without finning, you dummy. Relax. Let nature do the work."
Couple of questions, do I have air in my tank? Do I have air in my BCD? If no to both of those, drop that weight belt! If yes to air in my tank, inflate that BCD!
Inflate bc, drop weight if that is insufficient and/or not possible (leaking bc)
Make sure my BC is inflated. I'm missing something here
Inflate the BCD so that I float at the surface.
OK, most of you gave excellent answers here ... but don't hesitate too long before considering ditching some weight ... once you've ditched weights, even with a non-functioning BCD you will not sink. Always remember that weights are cheap ... and life is precious.
* What would you do if you were unable to equalize/clear ears on ascent (reverse block)?
Go down a few feet and try again. Keep trying. Try some more. Try again. If it doesn't work then surface in extreme pain. I've had a reverse block before. I was on a plane trip with my Mom. I couldn't get my ears to pop when the plane landed and OMG they hurt. I was crying and crying (I was 11 years old). They finally popped a little as we were waiting in line at Customs but they took about a day to get back to normal. Ouchie.
edit: just looking at that last part I realized it was backwards. Going from high altitude (less) pressure in a plane to ground (more) pressure would have been a squeeze. At any rate, same result, ear pain and one upset Mom.
Signal buddy, descend a bit and hang out there until ear clears.
I'd descend a little and try again, keep trying, chin wobble etc. I'm not sure how to fix it so I'd probably end up hurting my ears.
Assuming ample air remaining, go back down to where it's relieved and try again, but very slowly, doing the usual maneuvers. But I would not do this to the point where air was short. Short of air is one thing. Short of air AND in reverse block pain is quite another, and the block won't kill me.
When descending, pause or ascend a little to clear. If ascending, then I would pause or descend to clear
What if I have that problem while ascending and I just can't get my ears to adjust, and now I'm running low on air???? Well, if I get down to dangerously low limits of air for the depth I'm at, it's time to blow that eardrum and get the heck out of there. An eardrum can be fixed, no air while underwater can't be fixed. (right?)
First inform buddy of the problem. Then try everything I can think of to clear, air dependent. (Descend slightly and retry, flush with seawater and retry. If everything fails and low on air ascend and deal with the injury, better than drowning.
Descend until the pain goes away and try to equalize again by any means possible before tryng to proceed - yawning, moving my jaws, turning my head from side to side, etc. I'd sure hate the thought of still not being able to equalize and getting to the point where I have no choice but to ascend or learn how to breathe water.
Stop and wait a few seconds, gradually and slowly ascend. (???)
I know the question was about ears ... but another common source of reverse block is sinuses ... and a sinus block is extremely painful. If you get a block, first signal your dive buddy ... so they know that you cannot ascend normally. Give the "not OK" signal (flat hand moving like a see-saw), then point to where the problem is (ears or sinuses), and signal either remain here (flat hand moving back and forth), or descend a little (flat hand moving in a downward arc), and work on clearing. Once cleared, head up SLOWLY, clearing continuously as you ascend.
If you cannot clear, you will still need to go up ... it's just going to hurt like hell and possibly damage something. But eventually you're going to need to reach the surface.
One precaution I take is to keep a bottle of sinus spray in my car (dive kit). This isn't for use before the dive ... it's First Aid, to relieve pain and pressure if I have to surface with a sinus block.
Reverse squeeze is just one more good reason why we should leave the bottom with more air than “needed”.
This is something that's way worth pointing out ... any time you suspect you MIGHT have a problem is a great time to hedge your bets and dive more conservatively. If you are diving a bit stuffed up and think you MIGHT have a problem with reverse block, it's a great time to consider that you MIGHT have to spend more time than normal ascending ... and planning your gas reserves accordingly.
Remember, in scuba diving, what goes down MUST come up ... at least, that's the desired outcome ...
... Bob (Grateful Diver)
