ok, end of saga, maybe.
went to pick up the phone and found that along with the motherboard, apple also replaced the speaker, the face ID module, the screen, the camera (not sure which) and the battery. the genius could only speculate that they may have damaged the screen taking it apart, since she characterized this parts list as somewhat common. all of the parts have "physical damage" as the "reason" code. none of the exterior components were damaged when i handed the phone to apple. don't know why they would replace the battery except as a gesture of goodwill.
i tested the phone with the GPSTracker app and can preliminarily say the GPS behavior is better. while still not as good as the Xs, the location fix does not wander too far and i don't see the "lagging" behavior i saw before. on my test loop in the car, the old X averaged 18m accuracy while the repaired X averages 11m. of course i can't control for the positions of the GPS constellation so it's hard to know if the results really track the phone. i'll just have to keep using it with Maps/Google maps and see if it still thinks im getting off the freeway, etc.