Hi and welcome
#1. I’m not sure that there is a simple answer to this. I don’t play a healer, and I never played ESO. so I don’t have your reference points, but I think the best answer is to say that WoW has many levels of difficulty at end-game, and the amount of gear you have also affects the difficulty, so the technical difficulty ranges from vary easy to World First competitive difficulty, which is very very hard.
So there is a HUGE range of difficulty, from very easy to insanely hard.
Healing a decently geared group in Normal dungeons is very very easy. When you get to Mythic-Plus-Fifteen on Tyrannical week, you had better know what you are doing.
Healers usually say that their greatest difficulty is the idiots in their party who insist on running into damage
One thing I will note is that, at higher levels of difficulty of Mythic-Plus, healers are also expected to do damage when there is no healing needed. I don’t like this, but it is a thing that is expected in the more difficult content.
#2 "Are there parts of the game where you die and can never respawn until someone resurrects you? " Never respawn? NO. The game never, never, never locks you out permanently like that. All situations are recoverable. You can ALWAYS respawn.
In a party in a dungeon or raid where you die, you can’t resurrect UNTIL THE FIGHT IS OVER. So, if you die, you’re out of the fight, and you can’t just keep charging back in every time you die. In that situation, someone with a combat res can bring you back into the fight even though you can’t res on your own. That’s the situation you are probably thinking of. When the fight is over, win or lose, you can resurrect yourself then.
“if you’re playing a class that can tank well enough that if the main-tank dies you’re expected to replace and lead the group?” NO. This can kinda-sorta happen in Classic during a fight, but even then it would be rare. It could happen in Classic if the tank leaves while out of combat, but
In Retail, people’s specs are fixed for the duration of an instance, except for some really far-out cases you will never see. If the tank dies, you wipe, you all respawn, regroup, heal up, go again. Or you finish the boss without the tank, then the healer resses the tank, or the tank respawns and runs back after the boss is dead.
You will never be told to take over for the tank mid-dungeon.