Variant Breeding

As an example, I breed a Corsola and Arrokuda to get Galarian Corsola. Can I do the opposite? Breed Galarian Corsola/Cursola with Quagsire to get Johtonian Corsola?

I do not think it'll work that way. I tried to put an Alolan Sandshrew with a Pikachu and I did not get a "(!) This couple may produce something unexpected!" message. For giggles, I also did about 1k interactions and also did not receive any Kanto Sandshrew. So, I think it does not work in the opposite direction.