its meant to be kept alive until phase 3, the boss is unkillable until then specifically in case something like that happens.
and uhhhh iono.
definitely not basic party members, no. MAYBE you can bring them into specific places or they try to help sometimes or i make a town for them to do stuff, perhaps some secret events with some hidden items or a relationship thing, or pretty much nothing.
its up in the air
would be neat to bring them along as an assist of somesort... maybe they could count as a buff that does a random action from a set list? or a striker/summon? i've actually seen that last one done rather interestingly in annother game called slave girls rising where a temp party member would drop in and do an action then drop out during one segment of the game. either way even if its just a town or nothing it can still add to the story in the form of potental plot points. looking forward to how this shapes out trap.