I think I've finally realized a more practical situation in which ditching the lead makes sense, other than my previous example of when someone accidentally jumps in over-weighted to begin with:
Suppose I have an HP100 tank and a 7mm wetsuit, so I still need to carry considerable lead to be neutrally bouyant after the tank runs empty. At the beginning of the dive, I'll therefore be about 9 lbs heavy. If a diver was wearing 2x HP tanks, they might be close to 18 lbs heavy at the start of their dive. Now suppose that the BC inflator isn't working, or isn't inflating fast enough, or you can't get to it for some reason, and start sinking like a stone, but you're still at (or close to) the surface...In this case, it seems like ditching the removable weights (and aborting the dive) would be a reasonable move.
Your enthusiasm for this debate is really great to see! I'm glad you're thinking hard about this. You'll make a great diver! You have the right attitude. Just don't get too caught up in finding a way to make this come out the way you think it ought to. I say that gently.
Your conclusion is ABSOLUTELY correct: there is nothing wrong, and everything right, with dumping weight at or close to the surface (depending upon what that means). If YOU make that decision, don't let anyone second guess you. Positive buoyancy at the surface is the key to staying alive.
But here's where you're boxing yourself into a corner (and Scubaboard can be a little vicious if you are seen to be working too hard to make your point): you are pitching a scenario that is emphasizing your inexperience.
First, my 3500psi HP100's are only 0.6 lb negative at 500 psi, so they're no different from a pair of aluminum tanks for which I'm wearing lead to counteract their
positive buoyancy when empty. So the extra weight penalty of those tanks is negligible, because the extra weight of the doubles manifold means less need for lead. As in post #17, if you're neutrally buoyant, the
only issue is the weight of the air you carry. That math is (3500-500)*100*2tanks*0.08lb/CuFt_air, or 13.7 lb heavy. In fins, you can keep yourself afloat with that burden, at least for a bit. Yes, there
are kits that cannot be made neutrally buoyant without additional flotation, but those divers plan with redundant buoyancy. (Please don't come back and say, "but if I lost my fins, too...")
Next, you don't jump in the water without adding air to your bcd, and confirming your bcd integrity should have occurred as you put your kit together. So inflator efficiency is an imaginary horrible.
But, your conclusion was correct. If you didn't check and discovered that your bcd integrity was not there when you hit the water, dump your weights. So good on you! But you already were getting the right answer from several posters, and didn't have to come up with a doubtful scenario:
1) ditching weight at the surface is always okay, if you think it's necessary
2) ditching weight at depth is generally VERY inadvisable with a few isolated exceptions that should have been planned for anyway. But open water instruction still mentions an emergency buoyant ascent, and if you've got nothing else, then it's still okay to ditch weight and ascend as fast as you have to, to survive. Then off you go to the chamber, instead of the morgue.
But in every other case where you start neutrally buoyant (before you filled your tanks) and then ditch more weight than the weight of the air you're carrying, you'll arrive at the surface
positively buoyant, which can mean a runaway ascent as you near the surface.
Keep posting, and have a thick skin!
Safe diving.