Yeah, it's unfortunate that someone eventually decided to pick up the same DLC for a public project, but at this point it would be too messy to swap out the various characters. I also use some free resources anyway, so it's not as if there aren't more games around with some sprites, portraits and so on shared. Especially when I first started, I had a small budget, and generally I am trying to not spend too much money on assets (although I regret not commissioning more custom pixelart in places).
Just pretend he temporarily forgot. Eventually I'll add more spells for him, but since he doesn't fight, I wasn't in a hurry to pick a good list of spells. There is a bit of a story behind him AND he is also a recurring joke. If I have enough time, I'll eventually explore more about his backstory in the game.
The second encounter with Belthazar is indeed part of the walkthrough, maybe at the time it wasn't there yet? I only update the walkthrough after a release. I did forget to include the werewolf bit, however, so I'll edit the entry for the blackjack table.
Maybe calling it a walkthrough isn't appropriate then. The guide is more meant to be a "how do I get past this point where I got stuck" than a "let me guide you step by step through the whole game". Since the game is non-linear in many ways, going for the second approach was probably going to be messy anyway.