Thanks for the additional information. I really appreciate it.
It sounds to me like Bentley initially bonded with your mom before he came to live with you and is now protectively guarding her. Obviously we can't explain to him why your mother seems different than when she initially got him, but he can probably sense that there's something amiss with her and is trying to protect her from harm.
Normally, the person being guarded is the one who needs to really work with training, but in this case it's not possible...so what we need to do is eliminate the opportunities for Bentley to bite anyone in his misguided attempts to keep her safe. A couple things we could do:
1. Crate train Bentley. It's never too late, and you could actually even have the crate in your mom's room, or wherever he hangs out with her most frequently, and train him to go there when commanded. I know he's 7 and hasn't been trained to a crate before, but it is possible to do it...that will provide you with a safe place to put him when the caregivers need to work with your mom.
2. Hire a dog walker to come and take Bentley for a walk when it's lunch time. Obviously this is going to require a set lunch time schedule (which I'm not sure is possible for your mom...my grandmother could never be pinned down to a specific time for lunch), so if you think that's a possibility you could look into it.
3. Since Bentley doesn't seem to have an issue with you caring for your mom, you might want to consider getting him in a doggy daycare...then he could be out of the house completely when someone other than you is caring for your mom. He'd still have plenty of time to hang out with her when you're home.
I know none of these are terribly easy options, but as long as Bentley continues to try to protect your mom, you're going to have to worry about the risk of him biting someone. Keeping him contained and/or away from the situation will keep everyone safe and ensure you get to keep your dog.
I hope this helps.