Let's see, on regeneration, the zero point/state is the first time you show up in the Abyss.
Wounds that are not healed on entry, heal perfectly, but things that are scars, generally will not. However, this depends on how you get there.
This is true for people coming through a portal with their physical bodies. Stumps generally don't grow back with out some assistance (Unless it was just chopped off and bleeding--in which case you have active tissue to regenerate) It is possible to regenerate a healed stump that you come through with, but it takes conscious effort, time and mana, but you can grow it back, if you have the power/skill or help.
However, circumcision etc, stays like it is, and once lost, virginity can never be restored.
Even if you consciously force regrowth (which you could do for both, presumably)
If you form a new body via summoning/demonization/etc. All bets are off. You come through as the collective imagined version of those sending you (or just you yourself if you can do it yourself as one might think the Nyjyr Ennead did) So you could come up to any zero sate.
It's literally a land and air above/ground below swap, it appears that it's an infinite cylinder, but not sure that's true. We will see more of what happens in the opening of Book 4.
But, the rules of the plane that you swap to, apply to the newly swapped land, so the heat dissipates from over the Abyss, making everything nearby hotter as it gets cooler, standing near one in the Abyss is like standing next to an window air conditioner, at least for a while.
However, the swap lands do swap back eventually, either when another swap is initiated, or after a certain timeout, it's not a permanent swap. At least not the one that Exador does. Not sure if there are any permanent ones. What I know, I learned from talking to Gastrope' actually. It's really something that wizards do, more than demons.
I mean why would any rational demon want to exchange their lovely red, rocky homeland for mushy black dirt and green veggies growing out of the ground? Specially when they'd all wilt pretty quickly.