Steve is dead. Always was and always will be. Wesker's comment was a cruel joke to Claire. The only virus in the series at the time that could revive the dead was the G-Virus, and injecting it into a corpse only creates a Zombie.
1) Rebecca was meant to be featured in the
prologue artwork of BIO3.
2) Brad knew Nemesis-T targeted S.T.A.R.S. because it says it and
specifically pursued Brad for a long time.
3) Rebecca and Carlos did not have epilogues because the writer was not allowed to give them one.
BIO0 was in development at the same time as BIO3 and it was expected that FLAGSHIP would reveal what happened to Rebecca during and after the events of Raccoon City. However, they ended up making a strict prequel. Meanwhile, Carlos (and Nikolai) was written initially as a side-story character and it was stipulated that he would never appear in a main series game again. That's why those two have only appeared in side-games since. Though his writer did give us a possible explanation for what happened to Carlos: he escaped the U.S. via Mexico and probably got plastic surgery to change his face again.
4) Left town to live with relatives.
Joined one of the many chemical companies that scouted her pre-S.T.A.R.S. There are many possible explanations. We just haven't been given one yet.
She wasn't. They said they would like to feature her in a movie, but she wasn't actually
planned.