I personally think that sending invitations that far in advance seems a bit excessive. That leaves a very long time between the invitations going out and the RSVP date, and even between the invitations and the wedding, for people that may change their minds and want to come. Having said that, when we sent out our Save the Dates (18 months in advance) we knew where we were going, the date of the wedding and the week that we were asking people to come for the wedding (if they want to come for a week). For people to save money and know the time they have to book off of work, that should be enough in advance for those that are seriously considering coming to the wedding.
I think if you want to know who is going more than 3 months in advance of the wedding, make the RSVP date 6 months ahead, but just remember that maybe not everyone will be able to commit at that time. I think that regular ettiquette is out the window, so to speak, with DWs, but try to keep in mind what is practical for people ahead of what we ideally want (to know who the heck is coming so we can prepare!). For us, our payment deadline to book with our group is 3.5ish months ahead of the wedding, so that made a really great RSVP date for us.
As for a guest count for the resort, we had to book a minimum to book as a group, so we gathered our close family that is definitely coming and had them pay their deposits last fall (more than a year in advance) and we paid the rest of the deposits to meet the minimum, which is money that will count towards our trip at the end of the day. We don't think there will be a problem getting the minimum number of guests, but it seemed like a lot to ask for everyone to book/deposit over a year before the trip/wedding. We tried to think of it in terms of how far in advance you would usually book a vacation, and around 6 month seems reasonable, IMO!
Hopefully you get it sorted out!!