Finally some progress! After a few more requests, my accountant finally did fill out another Form 1116 for "Certain income re-sourced by treaty", but as he points out, there's no tax credit to apply to my return because I haven't paid any UK tax on this income (yet). This isn't entirely true as my wife paid UK tax on her US bank account interest income in previous years, but I only really care about the new royalty income and not <$20 of interest income. I didn't realize until now that the foreign taxes paid fall into different groupings matching the income. My previous accountants re-sourced the small amounts of interest income to the UK, but didn't ever apply any foreign taxes paid as they probably should have, and I also overlooked this as it was a small amount.
So, now how do I apply FTC for UK taxes my wife will pay sometime in 2021 (or early 2022 as they're not due until then) to our 2020 1040 return? I remember when we first moved to the UK my accountants advised us to pay an estimate of our UK taxes a year early to avoid this situation, and they mentioned that we could file an amended US return to fix the situation later, but that would cost us more in accounting fees. I tried searching for details on this now and can't find anything, as I'm probably using the wrong search terms.
(Edit: I tried some more search terms and looks like "carryback" pulls up some relevant results. I'm out of time to dig into the details right now, but it seems like after I make UK tax payments in 2021 for the Nov 2020 royalty payment and any 2021 royalty payment, and prepare/file our 2021 1040 return there will be excess FTC to apply to 2020 with a 1040X. Not the ideal situation but at least I see a path now to avoid double taxation.)