Also, I'm pretty sure that Deanna Troi was part of the Enterprise crew when it was first commissioned. She was killed by Armus on Vagra-II in an attempt to get an emotional rise out of the Enterprise rescue team (Deanna, being the only person on the entire crew with a high regard for sentient life).
I'd respectfully disagree here. While this is certainly one possibility in the alternate timeline, I think it's more plausible that the Enterprise crew never performed some of the same missions and never visited Vagra II because Troi's shuttle was never there. She could be anywhere in the alternate Federation
They ALL could be anywhere in the alternate Federation, but for some reason they're all on the Enterprise just as before. The only one missing is Worf, while everyone else -- even Guinan -- are on the Enterprise exactly where we'd expect them to be.
Their missions might be different, but Scifi logic means the parallels would be preserved. Troi's shuttle mission might reflect the fact that in the alternate timeline the Betazeds are considered expert infiltrators and intelligence operatives due to their empathy and telepathy; in this case, Troi is coming back from an FOB where she helped interrogate a captured Klingon officer and extracted some valuable information from him before the shuttle crashed and Arnus killed her.
Significantly, it explains why Tasha is on the Enterprise and Troi is not. It isn't that Tasha was died killed on Vagra-II, it's the Tasha wasn't the THE ONE who died on Vagra-II.