this is one of the same reasons why I kicked Mark's medicare coverage to the curb. It was a hassle from HELL to get anything paid for. Tricare for Life's coordination of benefits is in the shitter, and well...medicare is just a little bit worse. I seriously spent about 7 days on the phone with them to get some bills rectified. Who has that kind of time...
I can't advise anything for what's already been done about your coverage, but I would recommend that you IMMEDIATELY terminate your Medicare coverage and get back on tricare prime which covers you at 100% for everything.
It might be worth your while to go to the local DSHS office and apply for medicare, see if you can get any coverage for your 25%. See if you qualify for the "spenddown" if you are over income for any of their basic programs. They do go 3 months retroactively for bills, so if you can qualify somehow, this program will be of use to you.
Also look into the military's "program for people with disabilities" (your local tricare office should have info on how it works/how to apply), I don't know if they have any retroactive coverage, but if they do it might help you.
HTH