1. I think in the last Panel from Hell they said that drop-in multiplayers could create a brand new character then and there, regardless of where in the campaign the other player(s) are. They mention it around the point where they added another player with the PS5 pad iirc.
2. Characters can be reassigned amongst players, yes.
3. Unsure if non-Origin characters can be dumped into the camp.
4. Any of the party members can be assigned to any player it seems like.
