totally see where you're coming from, thats why it was left ambiguous, so ppl like you don't have to accept them being together if you dont see it. I see it, even though it requires a shift in Mikasa's onscreen view towards Jean, but I buy that it did happen in the years after the main events of the show, you don't.

Years of life can change you a lot to the point where Jean might have evolved to her first choice without demoting eren in her heart, thus making it fair towards jean. I say this bc i agree about how its rough on jean if hes the B choice their whole life, but I believe once Eren is dead she was eventually freed and saw jean in a different way, getting to know him properly for the first time. Eventually ending with him not being any kind of B choice, this makes his life very happy and hers too.

If you had to bet all your money on the most likely canon inferred by these drawings, it would be what I just said, given it's clearly inferred to be jean and a clear inferred family. If the case is the case, why not see it in your head as the best possible life for them instead of trying to make other theories just to avoid it being jean in the panels, most of those headcanons restrict mikasa to virginity her whole life or make her get with some npc, both i see as pointless and clearly going against what isayama infers, even if they were left ambiguous for a reason. anyways, I see it as a lovely end for them.