I wasn't really buying those theories either. (Although I personally would contest that because Ian picked the lock necessary for Belkar's plan to work, it's at least a plausible theory)
The active and honest qualifier is what makes this hard. Belkar has so many shenanigans but it's difficult to find people actually wanting them to continue.

But here's another few attempts, most of which are just as shaky.
Roy goes along with Belkar's claims that he's gay
a whole bunch of adventurers help him murder a kobold
V plays along with using the kobold's head as a litter box (IMO this is the best one I've come up with so far, but it still feels like a stretch)
And relatedly, the whole party encourages him to track down the stinky humans