I don't really think you should be able to bring as many alts as you want to quest maps...
It's basically the same issue as with distressing encounters, you can just bring as many alts
as you need to make everything trivial. With quests it's even worse because you can easily
keep returning after death over and over. It means there is really no challenge in ANY quest
and it makes impossible for a developer to set a proper difficulty level for the prize, because
players can go around it by bringing more alts or returning after death until quest is done.
I would make something like this: a quest location allows 5 player characters for example.
When a player character dies inside, the number of allowed characters in the location drops
by 1, if it drops to 0, the quest fails. So you could try to do bigger quests alone, but you
could die only 5 times, or bring 5 people and if anyone dies, the player is "out" from the quest
(unless someone else leaves the location to make space in the quest).
But I agree bringing players or even leaving and returning should be easier, as long as there
is still space in the quest. If you visit quest location you should see it on the world map IMO.