Thank you for working on this. I highly appreciate addressing long-standing issues and giving much needed support to our patrollers. My thinking is that this is just a button. The added-value of the button is so massive it justifies ignoring the details and making further improvements incrementally later on. I know I generally have a JFDI attitude but have you considered just doing it 😅?
I have one small annoyance for mobile diffs and it's when people post links to mobile diffs in Wikipedia and I'm mostly using Wikipedia in desktop, I have to remove .m. domain and also change Special:MobileDiff/ to Special:Diff/ so I can actually use the diff to my known workflow. But again this minor annoyance is nothing compared to not having an undo button in diff in mobile.
I suggest showing to people who have rollback rights (and add rollback button instead of undo). That way newcomers won't abuse this feature right away.