I’m truly sorry. If she is there almost every day then there is almost no chance of him bonding to you. He’s made it clear that he’s a one bird person. I think that even if she were to do pass over from her to you, he would only accept it as long as she was there or as long as he sensed no affection from you.
My ex’s CAG is the same way. Jake will let you move him around, feed him, clean his cage or any of the other necessary things but he will not allow you to give him scritches or play with or anything that is reserved for a bonded. Their beaks are wicked and they don’t hesitate to use them. I still have scars on my hands 8yrs later.
So I guess it depends on what you want from Toby. If you can be just a caretaker then that would probably work. If you want a bond I would say that’s out. If your MIL can’t take him right now, then maybe you can hold on long enough for her to be able to?
Have you tried having him step up since the MIL was there? Will he still do it for you?