If I`m getting your idea right you mean:
- Shared world / instance.
Where it will be hosted when the main host (who created it) is offline?
- Main Campaign disabled / Pure adventuring mode.
So it will be purely monster fighting?
How should monster respawn work?
It looks like something other than the current project. But I would also gladly play such "mode".
I think this mode would primarily be played as a group when everyone is online, so one player would host the game and it would go offline when they have to leave. Like when you host a game like Valheim and everyone can get on and play together. I think server hosting as an option would be nice but not necessary. I think the player's characters should be able to be imported between games. So I could make a character and play through the campaign then host a multiplayer game with that same character or a new character.
And for your second question, Yes, I would either say disable the main campaign and just allow open exploration and monster fighting. I think it would be possible to keep some if not a majority of the side quests built in as long as they aren't directly tied to the main campaign. As for monster respawn, I would suggest that when a party rests at an Inn or tavern in any NPC area that it would reset enemies.