^ perhaps the shadow realmis bach's own creation so he alone can't leave for extended periods of time before it perishes or similar?
well, that sure was a convenient reason for them to leave anyway.
Yeah I thought about it. Its very much possible.
As for the Senkaimon.
If Urahara would use official gate they most likely would end up in the middle of Seiretei. It wouldnt be good.
Besides maybe because he would be detected?
Alos Sun-Sung in the HM arc made it sound like its pretty normal that when someoething made of Kissi enters a spiritual realm its be turned to reshi.
There is also Orihime who got to HM through noraml made by Ulq garugana.
There is also hisagi who brought a Harley Davidson to SS.
As for Bach and the VR I believe the shadow realm is their own invention. I don't think normal quincy rules apply to them,after all Bach isn't a normal quincy hence his time limit on how long he can stay out of the realm. Haschwald mentions 'you' singular and not 'we' so that only relates to Bach and not himself or the other VR.
Yeah shadow realm is most likely their or Bachs invention, but still all Quincies use reshi manipulation and so this really must been made thanks to reshi manipulation and the same goes for the special shadow teleportation.
BTW Wouldnt people like Hime and Ichigo and Chad and Uryuu age like shinigami when their bodies are turned to reshi? I mean there barely any difference between them and normal souls when thei reshi..... If the Shadow reall is a spiritual reall I dont see why the VR wouldnt be able to live fore centuries.