You shouldn't have to export a file to do the kind of task you're trying to do. The file export is really meant for if you want to cancel your BandHelper account but you want to keep a copy of your data.
Also, not all bands would define "not possible" as one member declined the gig. Most of the bands I play in would still do a gig if 1 or 2 people couldn't be there. Since this could also vary from one gig to another, I think it best for the person booking the gig, not the software, to decide what is possible or not possible based on the band members' replies.
For the person doing the booking, would it help if the Accepted column showed something like +3/-1 to indicate that 3 people accepted and 1 declined?
For the rest of your bandmates (people not directly involved with booking), I'm not sure why they need to see who has accepted or declined every gig (although they can, by clicking the event edit buttons). It should be enough to see what gigs are confirmed. Once everyone has accepted or declined a gig, then whoever is doing the booking should either confirm it or delete it. Then gigs will only remain in the unconfirmed state for a short time.