-I suppose I could see it if Carl didn't decide to head to Ikaruga until later, but it would require something to put either Relius or Carl out of commission later to prevent having two of the same character playable, as was the case with the shift from Nu to Lambda (I don't see anything happening to Carl, but that's just my take).
-Litchi's story established that Linhua wasn't capable of fighting, though (I know you mentioned that these were gameplay theories, but it is relevant). Based on what Platinum told Bang, back when Trinity awakened, about his Nox being able to sever ties to the Boundary, I'm thinking he'll be more likely to play a role in saving Litchi. With no substitute for Litchi's gameplay to account for, I'd think Arakune would be safe for now.