How stable is it to do an official system transfer from a 3ds to a clean emunand on another system? has it been tried often? and vice versa? from emunand to 3ds on another system?

AFAIK you can't system transfer to emuNAND because the receiving 3DS needs to restart to finish the transfer, which puts you in sysNAND (and subsequently tells you to wait 7 days before you can try again). However, transferring from emuNAND to sysNAND seems to be fine.
So, if EmuNand to sysnand is fine. Then it looks like I can keep my 9.2 3ds xl or sell it.You can, I already did this when 8.1 was still the last update.