I'm glad if I've helped you think through a few things.

So Mom is willing to let son's girlfriend move in. How cozy! She really is determined to keep him tied to her apron strings. Wouldn't that be lovely to have her in the catbird seat watching every ripple in her son's relationship.

If you moved in there on the understanding that such a set up would be temporary, you'ld find that he'ld have one excuse after another as to why the two of you couldn't leave.

Between Mom and Son there are no boundaries. The two of them are committed to keeping it that way. She is way, way too intrusive in her son's life. You were very wise to reject the invitation to move in there. Your life wouldn't be your own.