Pulls aren't difficult as is. The only way people die on pulls is if they aren't up to gear or are completely ignorant of how to play their classes. Reset on battle start/end would not affect boss battles though. While the battle continues their cooldowns would need to go down properly. That means they would be affected.
You also have a weird understanding of what "balance" is. Based on that understanding of yours, balance is artificially extending the duration of a dungeon run. Because it's entirely possible to blast all of the cooldowns from every person in the party on a big pull even now. If that does not make a problem of balance, then letting that be done more or less frequently would not affect balance either. Balance starts and ends in a battle. The game is not balanced if the battle ends too easily or is too much of a slog for what it's meant to be (standard pull, boss etc). Shortening cooldown of Hallowed Ground is most certainly a matter of balance. But not because it would allow you to use it in more pulls. Because it would allow you to ignore more boss mechanics. THAT is what is balance.
I'm quite sure that a healer raising the tank does not remove them from combat. A person that dies have their enmity reset...that's all. They would need to revive at the dungeon beginning for that to work.
And there is an easy workaround. Have the reset happen at the end of battle. The mob resets or the person escapes by reviving at the beginning of the dungeon? Too bad, they don't get reset cooldowns. No option to exploit left there.
Not sure why you're trying to "explain" this to me or anything. Not like I even implied being against it. It would certainly be a "middle ground" between what is now and what I suggest. My post merely said that I think they should go even further than what was suggested.
And it's not like any content have to change, either. Really, future encounters could have slightly higher average damage and that's about all that would be required. To begin with, the difference between White Mages powerful heals and others having more tactical heals do not really allow much higher damage. Dark Knights also don't have pure invincibility skill, theirs is a pitiful "die a little later" skill, seeing as if a pull managed to bring them to 1 in the first place, it usually means that the healer simply cannot keep up anyway. So even if the deadly debuff will be cured by enough healing, the tank will still be at critically low health and be killed by raw damage. Holmgang and Hallowed Ground allows the healers to heal to full health to give some extra time before that problem. So those things need to be taken into account too.
Ultimately, my suggestion would mostly just speed up dungeon runs by a few percent and let people utilize their full arsenal more often getting better at using it overall. It would not really break anything.