[Avodah] two fictional sects

[David Riceman]

RMB:

<<I am saying the Benei Baishanim, after they left Beis She'an to go 
from being the Baishanim to the Benei Baishanim, no longer were 
following the minhag hamaqom of Beis She'an.>>

You need to reread the gemara.  It starts "Bnei Bayshan nahug ..." so 
your distinction between "Baishanim" and "Benei Baishanim" is 
untenable.  Furthermore the Rivash (#399 -- if you have the Mechon 
Yerushalayim edition its the second column on p. 606 -- alternatively 
it's cited by the Beis Yosef at the end of YD 214) says explicitly that 
the children of Benei Bayshan were still living in their parents' city.

Furthermore I found an AhS (vol. 9 YD 214:32) who says that you lose 
your city's customs after you've travelled 2000 amos away with the 
intention of moving elsewhere, and don't gain new customs until you 
enter within 2000 amos of your destination.  He doesn't cite a source 
but I'd guess Darkei Moshe YD 228:18*** citing the Mordechai.

As I mentioned before, the one possible partial source of your opinion I 
found is the Ran (Tshuva #48 - partially cited in BY YD 228 [p.467 in 
the Mechon HaTur HaShalem edition, otherwise look near the Darkei Moshe 
I cited above]).  He distinguishes between normal minhagei Ha'Ir and 
Heramim (particularly Herem d'Rabeinu Gershom), which "akarkafta 
d'gavrei rami".  It's not obvious to me (and it's a machlokes aharonim) 
whether he thinks this applies only to a man who married his first wife 
in a place where HdRG is observed or not.  Even if not, it's not obvious 
to me how it could apply to someone who was born elsewhere.  I know 
where my grandparents were born, but I have no idea where my ancestors 
were in the eleventh century when RG enacted his herem.  Even in the 
Ran's time HdRG was 250 years old.

Nonetheless you might want to check out the teshuva (the BY's citation 
is minimal, so it's worthwhile to dig up a copy of Tshuvos HaRan).

David Riceman