[Shin Henshin]

Well yeah, it's not a second season or sequel to SSSS.Gridman, but it being labeled as being in the Gridman universe doesn't mean it can't use moments or characters from it.

That story is done, but those characters can still appear in some for whether it'd be a one-off cameo or as a supporting role or what have you.

As for which world this takes place in, you can make just as much an argument for it being in the real world as you can the virtual. The reused backdrops could be a reference to locations Akane recreated in the virtual world herself and the places we see here are the real locations.

The Kanae dying in a kaiju attack theory can fit as well if they're rolling their Gridman tribute short into this canon and Takeshi does exist as Gridman Sigma, in which case a battle between him and a kaiju could have caused Kanae's death at the floodgate.

Plus the fact that Gauma is most likely a reborn mummy from the OG series would also fit that unless they come up with some explanation about him and the eugenicists being "reborn" into the virtual world. Who knows really.

All depends on where the story goes, but it's fun to theorize based on how the last series utilized its setting and history.