And of course another thing is whenever they want to support 6 player coop.
I'd doubt they'd want to. Too messy.
The good news is that they absolutely wouldn't need to.
Having a party that can potentially slotted UP to six memebers does not in ANY way imply that you need to extend it to the multiplayer side.
A multiplayer session would still be managed at most by four players (which honestly I don't expect to see happening very often) with two *potential* extra NPC being optionally controlled by as extras by them.