Finally I wait 24 hours to do the whole "I made a mistake" thing to try again. But she is gone. Lydia is now nowhere in my game. She's not in any of my houses. Fast travelling doesn't make her come back. She's not in Drasongsreach. She's just gone.
So then I asked Mjoll the Lioness to marry me instead. Evidently to the game, Lydia is chopped liver now, because she agrees. This marriage is finally succesful, but now Aerin is always at Mjoll's side inside my house. There's no way to make him leave, and killing him, even when she's not around or I'm sneaking, makes her eternally say "Get our of here. I'm in no mood to talk," for the remainder of the game. So I'm stuck with Aerin as a permanent resident in my house, which is already fairly crowded with a wife, a bard, an adopted daughter, and a steward.
I know the frequent excuse - one I've used myself on many occasions in defense of Skyrim and other Elder Scrolls games - for these issues is the unparalleled scale, NPC behavior, complexity, and player variability inherent to the game. And I agree that that's the case. But all of that size and complexity and detail, as much as I love it to death, is at times worthless because my immersion and suspension of disbelief and ability to roleplay are erradicated by something like this that sticks out like a sore thumb. And this is an old bug, one I recall happening back when the game first came out. Is it really reasonable for it not to be fixed yet?