The DiveRite fins are neutral in the water, so they qualify as light fins, Jet Fins sink like bricks, so they're heavy fins. That's why the DR's are one of the preferred sidemount fins.
You shouldn't have had that many issues with trim, though in a drysuit it is certainly much worse if you have a drysuit and light fins. A lot of this stems from the way the stab jackets lift is designed. You have a boat load of lift at your hips to pick the weight belt up, a floaty butt in the AL80, and almost no lift at the shoulders, so it's a recipe for nose diving.
In a BP/W the problem mostly goes away, and you see tanks sitting much higher. The twin cam band design, combined with how high the first cam band usually is, requires the tanks to be a lot closer to your head than a stab jacket where you can almost have the valve so low that the valve is in the middle of your back where you can reach it by going down instead of up. Bad news all around.
As for valve shutoffs, important for 2 reasons. #1 is actually valve turn on, lots of people jump into the water with their air not turned on all the way. Most DM's check for this, but it still happens all too regularly. #2, it's better in the advent of a freeflow to shut the tank off to prevent any damage to the tank if it aspirates water, same with the valve and first stage, just not a great idea to let them blow. Plus it is super irritating to deal with if you're sharing air on ascent. Makes a lot of noise, is a huge distraction, and for surface support can make them rush around thinking there is an actual emergency down below. A freeflowing tank or second stage at 60+ feet of depth creates one helluva roiling surface with expansion. Much safer for everyone if you can reach the valve and shut it off, especially if you are sharing air with someone and it isn't an emergency condition.
