Thanks again for your recommendation on trying to estimate brackets across different life stages. We've been going through these and your list above seems reasonable (based on tax/medicare rules of today).
As an update, I wanted to post good news that my mother is pretty stable due to some amazing immune treatments. She's a huge fighter and doing everything possible to beat her adrenal cancer. While no one knows what the upcoming months/years will bring, we are very grateful for this.
Additionally, my father and I were able to spend a lot of time running 2020 tax estimates (not an easy feat!), and just did a ~15% Roth conversion thanks to the lack of 2020 RMDs and the ability to harvest some losses in his taxable accounts to eliminate all capital gains and produce a $3k net capital loss for 2020 (and slight loss carryforward to 2021 probably), freeing up more 24% conversion space. Just a waiting game at this point to see how accurate we were, and I think the plan will be to continue conversions like this into 24% until my father has to file as individual (though RMDs are likely in 2021+ and will reduce the amount to convert by that much). As an individual, he will never fall below 24%, so no harm in converting now except for the cash to pay taxes on it and maybe the IRMAA-1 to 2 jump for 2022, which we don't even know would affect them both anyway.
I've learned a lot about how NIIT and IRMAA work, and how the "cliffs of IRMAA" are horrible for planning (and are based on MAGI, including tax-free interest and SS portions). Also, we don't even know what the IRMAA brackets will be yet, so have to guess the same or similar increases to last year... ugh. You give up some of the top 24% conversion space in order to stay below IRMAA-3, otherwise a couple is looking to pay $2604 of additional "tax" in Medicare B & D premiums alone (essentially another 5.2% minimum equivalent tax if you managed to completely fill the $276-326k space, but that will almost never happen and the effective tax rate on that premium is even higher). Since $1 over the IRMAA-2 level pushes you to IRMAA-3, we went back and forth about whether to build a good sized buffer or just go over it on purpose to avoid the situation where we went $1 over and "gave up" the upper 24% space for no reason and they still had to pay the IRMAA-3 level premiums. In the end, decided on a $7-10k buffer to stay below IRMAA-3.
Anyway, I've also obviously learned how important it is to avoid this situation ourselves in the future, so we will do everything we can to convert in the bottom brackets while we're younger!