It's a solid plan, I don't know if it'll work but I have two more weeks before I have to care.  I *think* the right answer is that the 2020 tax gets applied FIFO to the 2020 payments, then what's leftover should keep it's "payment date" character when moving to 2021 (so 5/17/21).  In reality, I don't know if the IRS computers know that.

Worst case I might have to cover a month's worth of underpayment penalty from 4/15 - 5/17 and we won't even know about that for at least two years.  Other than a few chronic "don't care about paying the penalty" clients, I almost always suppress the 2210 calculation and have the IRS send a bill.  There seems to be a threshold somewhere below which they just don't care.  I don't know where that is, but I'm not going to have a client volunteer $50 in interest if the IRS doesn't want it.