I mean Shadowheart specificly tells us that once the parasite will be dealth with, we will go our separate ways ...
You do some impressive mental gymnastics.
What makes you think that we will deal with the parasite in act1? Parasites are on which every narrative/mechanic contrivance lies. they are not going away - that’s an easy bet.
I would expect something more along the lines of characters not in our party at the end of the act1 to be taken under Absolutes control, or something like that.
