In the 6 months I was there, we had a few bad journeys back from day trips and the occasional dive called due to conditions not being viable in a boat, but it was certainly very rare and only happened around March. I think over the whole month there were only 4 or 5 dives that didn't go out.
As for the number of divers, it is down to the dive master you get at the time as much as it is the time of year. There are several mooring buoys on the shoal, avoiding other divers can be easy. Just speak to you dive master before you leave in the morning and I am sure they will take your requests into consideration when planning the dives. There are so many cleaning stations on the shoal that it is easy to have a dive in a small group knowing there are a lot of divers based on other cleaning stations.
I really would discount the weather factor though. It can be hit and miss any where in the world, I guess.
Most of the dive sites (70% at a guess) are around the island itself and are easily accessible. There are only a handful of day trips to various sites over half hour away, and even then the conditions have to be fairly terrible for a boat not to run.