I am sorry to hear about Mimi's new habit of scratching and biting/ingesting paint off the wall.
Ideally she would see her veterinarian for an examination and some blood tests including a complete blood count, biochemistry profile and a thyroid profile Some dogs exhibit pica (eating or licking/biting non-food items) when they are anemic or sick and we do want to make sure that she is healthy and there is not a physical reason for her behavior. She could be anemic, or suffering from metabolic organ disease, pancreatitis, or even cancer. This can also be a sign of nausea.
If this is lead based paint then lead paint tastes sweet, so it is attractive to dogs but ingesting it can lead to lead poisoning, anemia and brain damage.
At home to try and settle her stomach in case this is related to nausea you can give either:
1)Pepcid ac (famotidine) at a dose of 1/4 of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2)Prilosec (omeprazole) at dose of 1/4 of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers and should help her feel less nauseous, and thus less likely to chew/lick on non-food items. These medications are quite safe and can be used long term if they help her feel better.
If she seems at all nauseous you can try offering a bland diet of 1/3 boiled, minced, white skinless chicken or boiled, lean hamburger and 2/3 boiled, white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow and get additional fluids into her.
If she stops the behavior with the acid reducers and bland diet then try slowly reintroducing her regular diet by mixing it with the bland. If the behavior starts again on her regular diet then perhaps a permanent switch to a low irritant diet is indicated, such as the over the counter sensitive stomach foods made by Hills or Royal Canin or prescription foods like Hills i/d or Purina Pro Plan Veterinary Diets EN.
If she continues this behavior though she should see her veterinarian for diagnostic testing.
In some cases this has nothing to do with a physical problem. Dogs can indeed show some obsessive-compulsive type behaviors when they are bored for long periods of time or secondary to senility. They are naturally built to work with us. When we take their "work" away from them some dogs get a little strange with their behavior. This may wane on its own, but if she is healthy it may also help to give her more to do, such as more play time, hiding treats in kong balls that she has work to retrieve, or more walks.
Best of luck with your girl, please let me know if you have any further questions.