I have been thinking about your situation. You say that you will mostly need land only packages since your guests are making their own arrangements for air transportation. And then, some guests need the AI package with the airfare. Am I right? I understand that your guests' different needs make your situation more complicated. I have been there too and was wondering if this situation is happening because some of your guests want to use their "miles" toward a free reward flight. Here is want I found out about reward flights when traveling from Canada.
First, Fianc© and I booked directly with the resort to secure our date and we went with their AI package. So we used our Aeroplan miles to book our flights. I booked them the day they were released by AE (about a year from our wedding date). Since flights to St.Lucia are done with small aircrafts with AC (Airbus 319), it's been pretty impossible to book more than 2 reward flights (aka us) for our wedding week.
Also, we looked at the possibility to have guests traveling different dates so they can have a reward flight on another flight. We found out that booking the AI land only with AC vacations, Expedia or the resort itself for Anse Chastanet was more expensive than our group AI package with AC Vacations! Sometimes, the prices seems lower, but they not include meals, transferts and drinks. Once you add them, there is not economy really. So we told our guests to keep their reward miles and keep them for later! Even if we booked directly, all our guests are in a AI group with AC Vacations, including air transportation.
So, maybe it will help you to wait the release of the Aeroplan tickets before making a decision. If your guests realize that there is no availabilities for a reward flight or that it cost the same with the reward flight, it will be easier for you to recommend them a group package.
Each situation is unique but this is what happened to us. Hope it can helps!