So just a month ago you were even stating that if you moved out his way you wouldn't live with him, but get a place with your dog and cat.
Now you not only want to live that way but you want to move in with him in the hopes that he goes more for you than the person who he is with (who it doesn't sound like you have the most respect for with the comment of her being more than half his age).
If you don't want a polyamorus relationship don't have one. But don't try to trick someone who is in one to being monogamous with you.
Woodsmith: My husband
Merry: Tig's wife/slave
NT: Merry's boyfriend/owner
Elle: NT and Merry girlfriend
Umbra: Elle's Dom
Pet: Umbra's slave/wife, Elle girlfriend
Domo: Pet's submissive