I really dont think Tifa is too responsible for it. She could have said something, yes, but I think he was too deep in his delusions at the time. That isnt the sort of thing you can force someone out of without doing them some serious harm, perhaps permenant damage to his mind. She was there to guide him out when it was time, but I think she did right by not forcing him out.

I think Cloud was pretty much fully at fault. He let himself fall into his delusions, he constructed this deal for himself. No one else was particularily helpful at first, and some even served to further his delusions, but Cloud did it all himself.

As for meteor... eh, that seems to be more of a combination of Cloud, JENOVA, and Sephiroth. Cloud is at fault for handing over the Black Materia to Seph, but he didnt have to use it. Seph chose to use it, that's his fault.