I’ve read all or most of this discussion, and there are far too many things to pick on to answer anyone directly, so I’ll just toss out my general opinions on these things.
First, the three talents in general. They’re certainly not in balance, but I’ll go more to that later. Another point is that they’re boring, mainly because despite the name differences, they are the same on every character. I would much rather see some personalisation on them, unique ways of gaining temp health (or even otherwise enhancing your survivability). Ironbreaker gains a point of Temp Health for every blocked hit, Shade for every backstab… Stuff like that. They could even do something completely different: Giving Slayer an extra Wound point, allowing him to get downed twice even on Legend. But at the moment, they are the same on every character and even then there’s hardly a choice.
Health-on-Crit is problematic. There is a fundamental flaw in it that makes it nearly impossible to balance: It’s unreliable. Bounty Hunter is the only career who can reliably crit, and even for him it’s not constant or unlimited. No matter the actual crit rate, there will always happen a strings of on-crits when you really need that health, and strings of crits when you’re already full. What’s worse, neither can be predicted. No sensible amount of health given can compensate for that unreliability. Add to this that the crits usually kill your opponents too, and the problem gets enhanced. The only situation when the Health-on-Crit in somewhat better is on boss battles (both regular and Lords), where it allows health gain from the boss itself, instead of the (often scarce) trash enemies.
Health on boss kill is… Even less useful. It gives up too much for it occasional use of healing, especially since you need that health before the boss battle ends, and actual Temp Health can completely prevent that need for healing.
Health-on-Kill might well be too powerful, even when not compared to the other choices. Temp Health and its constant use is such a fundamental part of the play mechanics that abolishing it is something I don’t want to and can’t see happening. It could be toned down, though. The idea I’ve seen here that I consider the most sensible and interesting to change the Health-on-Kill is the differing amounts depending on enemy, but even that isn’t foolproof. Weapon choices (and somewhat less, breakpoints reached) affect the total gain too, as do some Career Skills. Tuning the amount gained from, say, Elites too high will give some careers and weapons again way too high potential, while tuning it too low, will cripple some. It would be extremely hard to find that balance.
TL;DR: Things need to be done to Temp Health and the three talents, but what they are is not obvious, and in any case will need a lot of care (and probably a decent-length beta test).