Should I contact Mom's credit card provider &/or debt collection agency to explain that she's on Medicaid, declining health & unable to pay?

When your mother dies her assets will stand for her debts. That is to say whatever assets she leaves will go to pay her debts. Until then there is honestly not a lot you need do. They can put liens on homes if they go to court and get judgements, but there is little else they can do. You will deal with her debts when she passes. Good luck. Meanwhile ignoring all this will not make a whole lot of difference.As you said, there is really no money, and they cannot get blood out of a turnip. Do not pay anything, as this will keep the debt "renewed" and prevent it going to a natural statue of limitation. You can say that she is in a nursing home and unable to speak to them, incompetent to do so, but do NOT ASSUME ANY PAYMENTS YOURSELF under any circumstances.
And it would be good to cancel any credit cards currently in her name if you have POA to do so. Upon her death you should notify Experian, et al (the big three) that she has expired, and they should not extend credit under her name.

While there is an amount on the cards I don't think you can cancel but u could freeze them. Do not call a collection agency. Once they have you info, they will be relentless. Just remember, you r not responsible for Moms debts.

Does she have any assets, like a house, to protect? Instead of letting it fester, she might want to declare BK.