Honestly, Strangeverse has a point. Chara somehow getting Mario to give up his soul is as valid a Wincon as Mario convincing Chara to BFR himself. It's such a unlikelyhood that it really shouldn't be considered. Was Mario tricked by Bowser before? Yes, he was, but doesn't Bowser have a higher intelligence rating than Chara? This is like saying L from Death Note get's tricked by Patrick Star- Maybe not on the same scale, but the idea is still the same. We can't just ignore the vast intelligence gap in favor of giving Chara a false wincon of convincing Mario to just give his soul up.
That being said, though I find this argument to have merit, I'm assuming this will be added ignoring this point, anyways.