So... is the actual problem being addressed really 'yo-yo healing', or is it the game not being lethal enough? Because most of the solutions are 'make the game more lethal' and if that's not the intent, it's just punishing players for a game design decision.
Good post. Worth dissecting...
OP seemed to state this problem: the PCs won't heal until AFTER hitting zero hit points.
Killing them for going down (when in-combat healing cannot possibly prevent it), imposing penalties that ensure they'll be doing down again soon, making it more likely they'll die if they go down again--none of it actually fixes the issue of people coming back up from 0, it just tacks on an all new, all different problem.
Punishing people to discourage a problem doesn't solve the problem is it's not one the punished person can't control.
The punished persons can control this: they can use their healing BEFORE dropping. But they don't, since that healing seems wasteful. Adding more punishment makes that healing seem less wasteful.
I'm taking 30 points of damage a round and the cleric is on a bad roll and giving me 12 back, it doesn't matter if I see the DM cutting a hickory switch and taking practice swings on the other side of the table, I'm going to go down to zero, either because the DM miscalculated (or worse used CR) or because the expected heals aren't keeping up with damage output--neither of which is my fault, but I'm the one about to have a sore red bum for some reason? That doesn't seem right.
So, negotiating, changing tactics, or fleeing didn't occur to you? That also doesn't seem right.
One thing I've not seen considered as a solution to whack a mole is to have healing spells provide a 1 turn defensive buff to conscious allies.
There are already spells that provide defensive buffs. If your party has a rogue, he can be scouting for possible enemies. If your party has a cleric that listens to the rogue, she can cast defensive buffs
before that battle starts. Like she should be casting healing spells
before you see the light at the end of the tunnel.
Besides not knowing when to run away, there's another bad assumption going through this thread: that enemies FIGHT FOREVER. Maybe your tank's max health is 50. Why are we assuming that the opponent will neatly survive long enough to knock off all 50 of those points? If the tank gets healed while fighting, her max health is effectively 58. Or more. Why are we assuming that the opponent will survive long enough to knock off all of those points, too?