1) The werebeast interaction uses the same logic the werebeast curse uses in-game. Seriously, I just stole the transformation part of the interaction from the vanilla interactions. So wouldn't it use one of the "werebeast types" generated in worldgen (see advanced worldgen)?
No. They are, for the most part, generated with a variety of, well, variations between each of the 10+ types a world will get. Why would it- no, HOW would it generate the werebeast types using the exact same ID? It follows the same logic that the raws do. Use the same ID twice and you get hilariously broken games. There's no WEREBEAST defined because they're given unique IDs.
2) If it doesn't, then why does the vanilla game come with no werebeast creature RAWs (unless I'm not seeing them)? By that logic werebeast curses would just turn you into a toad every appropriate phase.
Because that. They're randomized, not in the raws.
Because the werebeast creatures aren't special creatures
Definitionswise, they are. They are NOT, however, named WEREBEAST, and they are not in the raws, because they're randomly generated.
If you could open up the world.sav, you'd see all of the random definitions for that world. If you were to use one of the IDs from that list, I -guess- it would work, though that's a bit ridiculous.