My Mom chose a Medicare HMO which has gotten her into an unfortunate bind re: in vs. out of network doctors for care. Anyway to change this?

She thought she had to choose an insurance company by the Affordable Care Act deadline, so she left Medicare & chose a United Healthcare Regional PPO which has a 10k out of pocket for out of network doctors. It's a whole mess because now we have to look for doctors that take this coverage and as of today, found out from the business office for an oncologist we've already seen 3x doesn't take the insurance, a bill has accrued for co-pays and her cancer treatment now cannot start as planned b/c we have to find a new doctor and start all over.

Is there a way to go back to traditional Medicare or is she stuck indefinitely or at least until whenever a new enrollment period opens?

How can a doctors office tell us they take the insurance, we commit & meet with said doctor & outline a plan and now they say sorry, we don't take your insurance?? I was so aggravated at the level of incompetence that I just wanted to scream.