Well, we
did know we would be here when we chose our names.
One problem with the infiltration thing I just realized. If you let the svefra know before the quest, they'll demand his location and just charge him where he is, because, you know, they're zealous like that. If you don't tell them and try to defect during the attack, they'll just kill you anyway. Don't worry, we'll try to make it work, just be warned it may be different from how you initially envisioned it.
Obviously with stuff like this the earlier the better, though to be an accepted and trusted member of the crew by the quest start I'd say maybe an early Spring/very late Winter introduction. Latest date should be, say, the 15th of Spring. Later than that and I'd doubt you'd be trusted enough. If you do it earlier this year you'll be a worthy comrade by the time of the attack, loyal to by the entire crew, even Braten because he's so personal. You can decide the date, we'll see from there what your relationship with the crew will be.
When you're on the ship, I think there should be at least one thread where you're pirating with the crew. Comrades grow closer through experience after all. How you react is up to you, but it should register with Crypt what he's doing in the name of infiltration.