I wonder how many breaths I can get once I notice that the tank pulls hard.
I have heard experienced admittedly old time divers say to just dump air from your bcd, begin ascending and breath normally. Expanding gas in your tank will give you plenty to reach the surface. Thus my curiosity as to whether balanced regulators would result in less air remaining in the cylinder.
In your last paragraph I think you answered this precisely. 30-90 seconds seems a good buffer to quickly assess and make a CESA.
Conversely, when I read of CESA accounts they seem to convey that when that hard pull occurs, the diver should count on that as being the last breath and immediately start for the surface slowly blowing bubbles. (again, where alternate air is not reachable)
Again, the answer is complicated. It all depends upon how sensitive you are (vs. distracted) to a slight increase in inspiratory effort. With an unbalanced first and second, you'll see a perceptible change as early as 15 min before you're OOA, depth-dependent of course.
With a balanced first and second, and a little distraction, you may indeed be 1 1/2 breaths away from empty. Yes, you may get a breath or two more from expanding gas in the tank and tubing as you ascend. But don't make that part of your planning, practically speaking. Leave your reg in your mouth, make a CESA and take what you can get.
Your planning efforts are laudable at this stage in your diving. But I would recommend an ounce of prevention, as the better course.
1) Don't run OOA (duh
)
2) Dive with a buddy (duh
)
3) As you gain experience, consider Self-Reliant Diver training, not necessarily to start diving solo, but to learn more about the techniques you're pondering here, to KEEP yourself out of trouble, rather than GET yourself out of trouble.
If you're working solely on logic re: this question, you might conclude, "Why NOT go back to 1970's style diving, so I'll always get an alert from my unbalanced regulators?" And if that's the only consideration, you'd be right.
But now we have air-integrated computers to help you watch, and a habit of regularly checking our gauge (remember - we didn't always even have SPG's - hence the MkVII Honker 35 years ago). So things like ease of breathing become relatively more important. And there's nothing like the ease of breathing from a well-maintained, well-tuned pair of balanced regulators.
My 2 cents.
Good on you for asking the questions.
Happy diving!
EDIT: Referred to unbalanced piston first stages. Adding a diaphragm to the discussion made it too complicated.