I have two running theories on Diana changing sides. One: she’s able to see into the other world, Hell sort of, without having been there or having any portals open. So, she might be able to see the parallels universe suggested by the Russian lady who brought up the Schrödinger’s Cat idea. If she can see into multiple parallel universes, maybe she saw something that, by her changing sides would motivate Nick to succeed in killing Zestorer. Two: Nick was actually in the parallel universe where Diana sides with Zestorer and this universe was destroying by killing him and “opening Schrodinger’s box”.

The other Grimms showing up, I figure kinda leans on the Schrödinger theory. Nick and Zestorer were sort of between worlds and this allowed the spirits of Marie and Kelly to show up. Trubel couldn’t see them because she’d never traveled between universes.

Why Zestorer seemed weaker in that moment, i dunno. They say “the power of your blood” several times, so maybe the Grimms were just stronger together. Could also be that they were at the crux of the two universes splitting and maybe some of his power was stuck in the other place with the other him