I can also rather easily imagine the movies eventually confirming that the MCU is, in fact, Earth-616 since that number's iconic at this point, and the comics division then retconning what used to be E-616 was actually a different universe all along.
The comics don't have to retcon anything. Pretending that these different interpretations of an imaginary set of characters and ideas represent a shared multiverse is easy enough to do, but it's not actually real. Each version is still a separate construct, and if one of them uses the multiverse idea, it's using it in its own way for its own purposes, and that take doesn't have to agree with a different interpretation, even if they pretend to share a multiverse.
For instance, the comics tie-in to the Arrowverse's
Crisis on Infinite Earths includes a scene where the Arrowverse "Earth-One" characters briefly visit the events of the original comics CoIE, even though those comics' definition of "Earth-One" was completely different. They were under no obligation to reconcile the two, because it was just one story making a literary allusion to another story, using it to serve its own narrative needs and conceits, rather than an actual objective reality.
Of course, at least one Arrowverse production asserted that every Earth considers itself Earth-One, which is a handy enough way of reconciling the inconsistencies. (Although the
Crisis on Earth-X crossover implied the opposite, that "Earth-One" was universally recognized as the designation for the Arrowverse home dimension -- but then, it also claimed there were only 53 Earths, which later Arrowverse productions ignored.) The same principle could apply in the Marvel multiverse, with different multiversal observers using different catalog schemes in which various different Earths happen to be Earth-616.
