LF and 147: It’s a Machlokes Achronim. The Lomdus is fascinating and sometimes kind of strange.
DY: Presumably a Minhag that was established not Al Pi Beis Din can be removed by a B”D. It’s unclear if this is considered establish A”P B”D. I would presume that it is. I would also presume, though, that a Minhag that is established with a particular reason in mind can become Batel (by a B”D) if the reason is Batel. I think the Rambam says like that in Hilchos Sanhedrin. And there’s a long Tosfos on this… somewhere.
This probably falls into the Machlokes Achronim referenced above, but I would presume that when we are again Mekadesh Al Pi R’iyah the Halachah goes back to anywhere that can find out in time will keep one day (which is what the Halachah is based on now, contrary to popular opinion; it’s not E”Y vs CHU”L, it’s E”Y and Aratzos HaSmuchos vs CHU”L), which will presumably be everywhere.