If one of the FO 3 game guides is any indication, you are already mistaken, as in that they canonized the LW disarming the Megaton bomb. For now, I don't consider it canon, because it didn't appear in every game guide, but I do think that quest, and the Replicated Man quest, will be decided in 4. I also wouldn't be surprised if the Reilly Ranger quest canon ending becomes known.
I would also say that blowing up Megaton isn't really a "neutral" action, pretty downright evil, and having a "good" LW who saves the CW doesn't make much sense to also have that LW be a mass murderer, or a slaver.
If Zimmer, Armitage, and Harkness are not in game at all, I would say it is safe to assume Zimmer is dead, his body chucked into the Potomac.
I really don't understand why people want to cling to what they did in a game(multiple different times more than likely) 7 years ago. Boggles the mind.