As for time travel, we don't know if they're gonna use it to wave away stuff, especially now. I mean, they sure didn't hand wave Tseng's bizarre as hell vampire aging, lol. Right now the time travel element is a mere fantasy gimmick, we just don't know of what sort. It's new but it's also not beyond the realm of Final Fantasy, particularly spirit energy-lifestream-gaia theory which is the foundation of FFVII and it's cousins
The issue for those taking issue isn't so much a notion that time travel doesn't or can't work within the existing metaphysics, but rather that it probably doesn't work within the existing themes. That's a concern I fully understand, as I also remain quite concerned about where that thread will take us.
As I see fiction -- and sci-fi and fantasy fiction in particular -- it's like a good mixed drink. Those don't ever actually require more than three ingredients. Now, you may go somewhere that puts four or five ingredients in a drink requiring less than that, but this is so they can charge you more for it.
Successfully constructed fiction generally works when it orbits just one or two "let's pretend" elements strongly woven throughout. Maybe three at a risky push. More than that, and you're almost definitely losing thematic direction to your narrative.
I can't look at "The Lord of the Rings" and conceive of any scenario where time travel would elevate that setting or the cohesion of the themes depicted there. Likewise "God of War" and KH. Of these three franchises, however, only LotR is without the introduction of time travel shenanigans, and thus it is the only one not irreparably harmed by the same.
If you were halfway through a sequel to "The Bourne Identity" and Bourne's adult son from the future suddenly showed up, you'd understandably be concerned that the narrative was going to lose you even if the nuts and bolts elements were played to internal consistency.
Plot devices outside real-world territory require one part internally consistent mechanics, one part cohesively blending those devices into themes.
It's damn near impossible to throw everything at the wall and still get it to work. The MCU may be the only case of successfully pulling this off in recent memory, and I still don't know how they did it. Growing up with the comics, I just took mutants and super soldiers and magic and aliens and vampires and time travel for granted, but each time I thought the movies were going to get one step too far afield with their plot devices for general audiences (e.g. Thor, the Guardians of the Galaxy, Dr. Strange, time travel in "Endgame"), they stuck the landing once again with thematic structures that carried sufficient resonance.
The developers of VIIR could stick the landing, and I really hope they will. I just don't blame anyone who feels more trepidation than confidence.
and yeah imo remake may as well have white power ranger and Sephorita Repulsa commanding sexy versions of weapons attacking the party. 20 years, multiple sequels in an established universe, and only now a game-changing element such as time travel is introduced? If you're going to pretend that everything before mattered, then commit to that. Don't just use multiverse time travel as a way to be vague about how commited you are to to continuity of your own project. Come on.
Of the many charges fair to level at the remake or its developers, I'm not sure this is one of them. By that, I mean Nomura was clear before the remake released that it was not meant to be OG FFVII Final Mix. He said it was a separate story from the original in a separate universe, and he hoped people would continue to play both.
Except that doesn't matter. The entire point of how they added it, however ungracefully, was that Cloud would have no knowledge of it, and therefore would not be included in his renditions of the story.
I don't think that was ever the point, though. Cloud and Tifa were fighting Genesis's copies while Genesis himself went inside the reactor -- which isn't particularly inconspicuous. Also, Cloud has been shown to have verbatim knowledge of what else went on inside.