Wow. That is a cool. My limited experience working with single-threaded game-stuff tells me it is exceptionally hard to port stuff that is written without threading in mind, to multi-threading. Getting the behavior to stay the same while still actually getting better performance requires some really deep insight into how stuff works in the program. On a (program-)global scale. Mad respect if it works out. This should make huge maps or huge fortresses possible.
I haven’t yet played the steam version (it is on my todo-list though), but sank quite some hours into the “legacy” version. It can become laggy if you play on big maps with a lot of dwarfs/critters etc on it. I am excited to have even more stuff possible in this already very complex and huge game.
That fully depends on your definition of what the “reversed” state is. Will ecosystems adapt to the changed climate? Yes. Can temperatures even drop again? Sure. But species lost are gone forever. Ecosystems destroyed will never be the same again. Different ones can develop with enough time and stopping the human interference… But that is hardly “reversible”. That is… natural systems covering up the damage. Going on. Not reversing it.