Oh dodie, I'm very sorry for your loss.

As the executrix of the estate, you personally are not responsible for debt UNLESS you are a joint account holder or you co-signed for the debt. If you are not a joint account holder or you did not co-sign for this account, this should not be showing up on your credit report.
However, the creditor can contact you about the debt since you are the executrix of the estate. They are allowed to "go after" the estate for the balance due.
If you are not a joint account holder or you didn't co-sign for the card, I would talk to the probate attorney again and get his/her assistance with resolving this. S/he will probably have to help you send the proper documentation to both MNBA and the collection agency. The estate needs to settle the bill, and the entries need to be removed from your personal credit report (most important).