I think it would be easier for Jerome to survive than a lot of other characters. While he may have been last seen being pushed around by the sinister duo, he was in the lobby at the time. Once he smelt smoke, it would've been easy to escape.
I think the core issue is where each of the key characters were at the time of the fire. Realistically, everyone but Justice Strauss (although if you read my ages old fan-fic, JS does a little building surfing until it was safe for her to jump into the pond and dive to the library) should have had enough time to get out - the fire was not made to be massive from the start.
Mr. Poe: Dead. Lemony Snicket states he dosn't know what happened to him. But Mr. Poe would have returned to his normal life (based on his personallity, the fact he has a wife and kids and a job). All other characters would have had a reason to leave their previous lives except him.
I think that was more acknowledging the possibility that Mr. Poe might die from a harpoon, or might not; since Snicket doesn't know what happened to him, it's entirely possible. In addition, possibly part of the reason why Snicket wouldn't know what happened to some of these prominent individuals is that he never looked into their lives at all, so once they stopped being involved with the Baudelaires' story, he never had any reason to hear of them at all again. My personal take is that the best reading is that any character could or could not have died in the fire, depending on simple personal preference; that the possibility exists for any of the characters to have died or survived.
No, some surviving theories sound more realistic and some of them sound less realistic. I want all Justice's Strauss's fans to forgive me but I am almost sure she is dead. But Frank could survive because he probably believed them and he was in hall when the fire was in progress, so it is very possible that he survived because he had enough time to escape. For the same reasons, I think sinister due and Jerome could survive. I don't think Geraldine survived because I don't think she believed Baudelaires. Mrs Bass definitely survived. I am not sure about freaks. What do you think about freaks?