He doesn't really show that it is possible to believe something intentionally, though.

I think it's certainly possible to make an intentional decision to train/brainwash yourself into believing in something.

Pascal's wager is interesting but it makes a core assumption that makes it laughable - namely which God/religion/deity do you bet on?

The positive sum that the wager demonstrates in a belief in a God is nullified when you divide by the number of Gods/religions/deities in existence (many thousands/millions). Indeed - if you factor exclusive Gods/religions/deities that condemn separate beliefs - the wager goes negative since holding many beliefs that negate each other, with the addition of trying to believe them all, will cost you much more than just not doing so.

This assumes pure "afterlife" expected probabilistic value - I'm ignoring societal pressures which would eliminate this problem of many Gods/afterlife and replace it with "Believe in what we do - or die!". Then it's not Pascal's wager any more - it's believe or die because others force you to.