If we go with the “he’s already wearing the suit” option, perhaps he fought the other pupil and they both sustained substantial injuries before Palpatine intervened. With it left ambiguous who truly lived, Vader would seek out Obi-Wan
Yep, that’s pretty much how I’ve always wished it had gone.
I don’t think it ever would have been possible to do the prequels while keeping Yoda’s appearance a secret, though. And that’s not a big enough reveal in Empire that the audience knowing who Yoda is before Luke causes that much of a problem IMO.