The characters believe Harry and Jesse are destroyed. HOW? Frost/Caitlin and Cisco weren't Paragons. Only the Paragons have first-hand memories of the old timeline. Nobody else should. So how can anyone remember Harry and Jesse if their world (and them) never existed? And how can there be a holo-recording of Harry if he never existed? And what's with Cisco saying he doesn't remember getting his Superman shirt? Since he's a part of the new timeline and Earth-Prime, of course he should remember getting it. He's not like Supergirl who found herself in the new timeline and was surprised by the differences. Cisco is a part of the new timeline. None of this should seem off to him.
Even the handheld devices that let them jump from one place to another, tech based on Cisco's Vibe powers... If he remembers them working, they should still work. He says the laws of physics have changed. FROM WHAT? From a timeline he was never part of and by rights shouldn't remember? The laws of physics should seem the same to him as they always were, and if he remembers them using the devices (I forget the name) to more or less teleport, those devices would have to have been based on Earth-Prime physics, and THEY SHOULD STILL WORK.
Nash. Do I need to say more? I will anyway. Nash shouldn't exist. There ARE no other universes? (We know better, but the characters seem not to.) Then where did Nash come from? Who is he? Is he a leftover from the old universes, or possibly someone who got shunted from one of the new realities when the new multiverse came to be...which would make them a missing person in their own world? Like Beth on Batwoman and a bunch of people who showed up on Supergirl?
The fact the writers are apparently so lacking in understanding of what's happened here and who knows/remembers what... It's spoiling the whole thing. I can't figure out anything within the story that would explain these seeming glitches in continuity, so it all comes down to people in the real world being just plain SLOPPY in how things are being presented.
If anyone here DOES have a decent explanation for things like Cisco, the laws of physics thing, and so on...PLEASE share your ideas here.
Martian Manhunter used his telepathy to restore people's memories of the old timeline. The rest of it, I don't know.