I'm so sorry you're dealing with this! With all of Bella's eating issues, I fear that someday I will deal with all this as well. Hang in there.
My first reaction was going to tell you that I think this is similar to kids. When I brought my second son home from the hospital, Ethan loved him to death because it was really exciting and he didn't quite get that Brayden was here to stay! LOL After a week or so, it occurred to Ethan that he was no going to have to share his home, his life, him Mom and Dad with this baby and felt very... concerned about what that meant for him. Of course he overcame that quickly and they are the best of pals, but maybe that is sort of what's going on with Lola? See, Bubba already went through this, and that would explain a lot of why he's trying to "stay out of it", but Lola's life is changing and she's probably figured out by now that Mia is not just "visiting". Some time to adjust will help I bet.
After reading about Mia's personality I'm sure that only aggrevates the situation, because Lola sees that fiesty personality as threatening. Mia will definitely need LOTS of obedience (I had a dog like her and at 8 weeks she was just like that. She turned out great, but I had to constantly remind her I was alpha). With Lola, find that boundary of what you feel comfortable allowing Lola to correct, and once she crosses that line with Mia (i.e. about the shake her to death!) stop it. That will help with consistency as far as teaching Lola that she's ok to teach and correct Mia, ONLY to a certain point and becuase you're the ultimate alpha, you decide what that is. Chances are that Lola may be able to help settle Mia's fire down a little if Lola herself can adjust to the idea of sharing her world.
I think Butt's Mom is also right about spending quality time with each of them... whatever they most enjoy doing with you. For Bella, it's cuddling on the couch. That way even though Mia's there, they still get some special time.
This is all coming from a person who has not owned more than one dog at a time on my own, so take it for what it's worth! I'm no expert that's for sure, but I always have an opinion! LOL
Good Luck. I hope things work out.