Baldur’s Gate 3 – Can You Remove a Co-op Player’s Character from the Party if They Stop Playing?

Yes, you're able to do each Origin Character's story quest when they're companions. If you play as one instead, you get a few extra internal monologues and minor things (such as Tara showing up in camp if you play as Gale), but that's all.

The exception to this is the Dark Urge origin, who is never available as a companion.

You can find his corpse inside Orin's bedroom though.

• No. Dishonoured mode is basically a custom mode. Your game will simply no longer be in Honour mode, invalidating the Foehammer achievement progress. (See the Patch #5 notes here.)

• The game will save even when Alt+F4ing out, so that will only make it harder to survive a specifically perilous situation.
  A way to bypass this (at the moment) is to Alt+Tab and quit the executable using the Task Manager (Ctrl+Alt+Esc). Restarting the game will allow you to load your last save file, and your playthrough should still qualify for the Foehammer achievement.