I would deduct 9/84th of the new mortgage. 84 months is mentioned in publication 936, and therefore I think in case of audit, relying on months rather than exact number of days is not likely to raise an objection (unlike certain other cases where the instructions require calculating to the day.). To get technical, the new insurance policy (which is insurance provided to the bank, in case you default) was provided beginning on the closing date, when the bank became obligated, so the insurance start date would be 3/30/2020, and while you could bump up your deduction by 0.04% (1 extra day/2555) I'm not sure it's worth it. You certainly can't start on 3/1 since there was no insurance in place until 3/30.
I would deduct 3/84ths on the old mortgage for the same reason. While we can certainly argue for those extra 6 days, we can also argue against them for the fact that, once bank B assumed the role of primary mortgagor on 3/30/2020, bank A had no insurable interest in the home, even though bank A did not record the final payment until 4/6. For the same reason that you can't deduct the lost 77 months--you can't deduct a future payment for an insurance policy that does not exist--then you probably can't deduct those 6 days.
Either way, this is just nibbling around the margins and doesn't change your deduction in any useful way, maybe you could buy an extra coffee on the difference if you pushed the margins. For mortgage A, either claim 3/84 or 96/2556, if you want to squeeze every penny (2556 because there's at least one leap year in there). For mortgage B, either claim 9/84 or 277/2556, if you want to squeeze out one more day.
The main reason I wanted to speak up was my realization that you can count (in your case) the month of April, even though your first mortgage payment was in May.
