I doubt they’d do that. Big problems if they had. But the whole plot was likely written well before that all happened to Hayden, plus the amount of time it took to shoot, and then edit and add the CG with the powers and whatnot, that whole plot idea was probably come up with long before what happened with Hayden. Just…

Hiro was in the hospital at the time and his power no longer worked because of the evo-cloud thing. And anyway, maybe Claire did wake up, but after everyone left. Theoretically, she could still be alive, but of course they can’t really do anything with that, since Hayden didn’t come back

I can’t answer the first three but I’ll give the rest a shot, mostly because those are pretty self-explanatory, or evident if you paid attention...