I jumped through these hoops earlier this year as executor for my father's estate, and DPoA for my mother who was in a nursing home with dementia. All of their joint assets were inherited by my mother. No brainer, right?
Many of the big banks (BoA) have stopped doing Medallion signatures guarantees because, as noted above, they're on the hook for the value of the transaction.
I had three bank/credit union options, and discovered that at two of them, the person with medallion signature authority didn't know the rules. Mother inherited a 7-figure IRA from Dad. I presented my Letters Testamentary as well as her DPoA. The doofus at the credit union where we all had accounts said he could only issue a medallion cert if I had the same 7-figure amount in the credit union. Uh, no.
Bank number 2 -- where they had a small joint account -- wouldn't do the medallion signature guarantee unless one of the account holders -- one dead, one in a nursing home -- appeared in person to sign. Uh, no. And anyway, they were guaranteeing my signature, not my parents'...
Third bank, BBVA-Compass, was perfect. My parents had about $25K in accounts there, and it was just a matter of finding a branch with someone certified in Medallion signatures. Once found, I visited her many times.
MORAL: some banks, like BoA/ML have discontinued medallion signature gurarantees system-wide. For other banks which do not have system-wide embargos, if you get a doofus answer, try another branch: the people there may know what they are doing. This is the branch-bank equivalent of phoning an investment bank during another shift if the first person you spoke to was clueless...
Regarding PoA after death: yeah, PoA stops at death. But since my mother inherited everything from my father (and he had a will and I was appointed Executor with Letters Testamentary) we never set up an "estate account." I just kept open an old account with both of their names on it, and when small checks payable to him came in, I endorsed them as "agent/DPoA" and deposited them because the bank staff didn't know he was dead.