This is definitely not a good idea until you secure housing. Your partner's credit score will drop into the 500s as soon as she takes out a loan. There is a sub called r/mechanicsadvice. I'd suggest posting in that group describing exactly what the transmission is doing and see what advice you get. A lot of automatic transmission problems can be solved by simply changing the fluid.

What makes you think the transmission is going out?

I don't see the lawsuit going anywhere. The drug company will say it was the mental illness causing the irrational spending as opposed to the medication. Is it worth the time and effort to go back and see where the money went? If it's gone, it's gone. Why not just start now and figure out your debts, expenses, and income, and go from there? If the debt is insurmountable, bankruptcy is an option.

(I speak from experience with this. I'm bipolar and it's really common for people in the hypomanic or manic state to spend too much.)

I am sorry for your loss. One thing you need to do is get multiple copies of his death certificate. This will help you as you negotiate everything.

One quick thing I thought of. If your dad would give you power of attorney, you can then speak to Optima and the IRS on your dad's behalf. POA doesn't have to be permanent. Your dad can give you POA for a short time to deal with all of this and then revoke it when it's over.

No advice except to say I am sorry you are going through this. Minimally, they should return your security deposit. I live in an apartment owned by a large corporation that owns many complexes and they are really hard to deal with. Just keep at it. Perhaps go up the food chain and see if you can get better results there.