1) If you take Lynch in Neverborn you can only hire ten thunders models with the darkened characteristic (and if memory serves your limited in number), are mercenaries (like the Oiran) or that are dual faction with Neverborn (such as Mr Graves)
2) Whenever you hire a model out of faction that can take upgrades (so a henchman or enforcer) they use the upgrades available to your declared faction.
So no sorry you can't take Kang with Neverborn Lynch though you could with 10 thunders Lynch.