Unlikely that is the case at all. The reason we have Alolan forms is because most Pokemon adapted to living in different ecosystems/climates in Geodude/Graveler/Golem, Exeggutor, Sandshrew/slash, Vulpix/Ninetales, and Marowak's cases. Some were changed through human interaction like Grimer/Muk, Meowth/Persian, and Raichu.(If you want to take Pancakes seriously)
Also, different final starter evolutions is very unlikely too.
I don't know, this is supposedly taking place as an alternate story, so its very possible that this green flash concept very well may have an effect on some Pokemon.
I doubt it only exists for Rockruff to evolve in the hypothetical new form.
Unlikely to get brand new Pokemon, so yeah maybe the day/night evolutions of Pokemon won't be affected, like we won't get a strange Twilight Eevee evolution.
But I do think its at least plausible we could get regional variants of certain Pokemon with this hypothetical new form, after all how do we really explain normal non Alolan Pokemon evolving into regional variants just because of them being in Alola.
Maybe we could get something like a regional variant of Gliscor, that evolves from a normal Gligar like this. Gliscor evolving from Gligar at night with the razor fang. Maybe it'll receive a regional variant similar to Marowak but under the conditions of Twilight/Green Flash.
Maybe there will be a 2nd Alolan Marowak that is a result of this plausible new game "mechanic"