"I spoke to the Editing team (who are working on Discussion Tools) and it looks like they might actually spend some time on editnotices sooner than our team would have got to it."
Does this mean editnotices in general, or editnotices in DiscussionTools? In case of the latter, I really don't believe they should bother. But if they could spare any resources at all, I could probably use them..
"Does that match your experience (on desktop or mobile), or do you find yourself using the edit window when undoing edits?"
I suppose this is partially why I request the rollback right on wikis where I edit a lot. Fire and forget, love it. When I undo, I actually do perform partial or altered undoes fairly often. Most of the time an undo is just an undo though. Occasionally I'll preview before publishing.
Perhaps the way the "thanks" link works could be a viable approach here? Silently check if the edit can be undone and give inline choices to instantly undo, open the full editor or cancel. To make it more responsive, show the text immediately with the instant undo link greyed out/disabled. Enable the instant undo link if the silent check comes back positive. When clicked, the instant undo should ask for an edit summary in a popup. This would be nice on desktop too!
And of course, preventDefault() and an href to undo in the full editor on the original link so especially desktop users can still quickly go there with a middle click.