Nothing happens, the spell slot is expended, and the caster perceives that nothing was done to the target.
The second caster would see that the target was unaffected, but would not immediately know why.
Xanathar's Guide to Everything Offers Guidance on Invalid Spell Targets
XGtE p.86 :
If you cast a spell on someone or something that can't be affected ...
Yes, though it's a bit expensive
This does seem like a nice thing that your group could do for their lost allies.
I've never seen a group do it, possibly because it's quite expensive. But it makes sense that a cleric might want to revive their lost friends, and it could be a nice roleplaying moment.
The newly revived character would become an NPC
Yes, if their soul is available and willing
You have to remember, that druid has already spent a year in their respective heaven. It might seem like every character that ever has or ever will die will be able to come back from the dead, as long as you find enough spell components to do so, but these characters might not want to come back anymore. If you've ...