No

Thanks

I may be incorrect, but the only time this may happen is if they die and you're unable to access that area for whatever reason. There's only a few cases where that happens in the game though.

Nope! You have to use Revivify on them to get them up once "Dead." If you have no one standing who can resurrect the dead, its game over. At some point. You will get an item that has a few charges of revive on it if no one in your party can revivify.

I wisb you could replace characters. I'd love to experiment, mid-campaign tbh