First I would like to say that I was a little shocked that I was able to guess about the magic but now that I think about it the both are getting the player too help them do magic.
1 A way that it the other transformations could work is a centaur or Minotaur situation. The spider it could be like the last two or the player could grow two extra set of arms or one set of arms and one of legs. The golem could hardly change the shape but it would make the body grow rock and Crystal like extension's and Hardin the skin.
2 I want to suggest vore and you may have shown that it is a possibility with the guardian and the fungus pod but if it doesn't work for you as a preference that is fine the game is great without it and in my opinion would just be better with it but you are the one making it.