I would think you could make most of the arrangements with 3 or 4 days notice. One hangup might be the flowers as Floral Fantasies wanted significant lead time to get the flowers we requested. I'm sure they always have something "on hand" but it may not be exactly what you want.
We did not use a travel agency. Only about half the people who said they would come actually made the trip. Cost can be minimized by staying in less costly hotels and eating at local restaurants, but it takes a different kind of traveller to do that. We provided a real "Jamaican atmosphere" for those that were so inclined. A few couples wanted the AI hotels, paid a higher price and missed the real Jamaica. Even my parents (both in their 80's) stayed at the eclectic Blue Cave Castle and we took them to the local restaurants and a day trip to Black River and YS Falls.

Regards,
Bob