The only times the game assumes you are in a party are during the "siege" scenarios (attacking a fortification and what not) or when going against a massive threat...the trials usually. You are most certainly "in a party" for Final Steps of Faith, for example. Heck, you even "gather" the party way before the actual battle...and that party you fought with was nearly entirely wiped out by the story, even...

Basically, don't expect there being any logic to an MMO. By the fact of it being repeatable, there is none possible. Except if the game explicitly made use of time loops letting the character do something with a definitive result over and over again, changing the outcome through changing the decisions...but it's neither here not in any other MMO I know of.