Suggestion 1: I'd keep monitoring the el Niño reports, because it is supposed to end in the coming months and then trasition to La Niña, which brings cooler waters and is usually quite favorable for diving for places like Cocos and Galapagos.
Suggestion 2: There are generally two monsoon seasons in Maldives - the Northeast and the Southwest monsoon. You are mostly correct in that the season for diving the Central atolls and the so-called Best of Maldives itineraries that include Ari Atoll is during the Northeast monsoon early in the year, altho I believe that some liveaboard operators still find the way to dive in the sheltered areas of those Central atolls. But there is more to Maldives diving than the Central Atolls.
During the Southwest monsoon, which encompasses most of the months in your July-December timeframe, some of the diving shifts to the Northern atolls, among which is Baa Atoll and Hanifaru Bay, whose claim to fame used to be (and might still be) the Reef Manta cyclone. Having said that, I went there in September 2010 (which is many, many moons ago). I don't know what it is like nowadays but if this even stirs a bit of curiosity, I encourage you to research it.
In addition, I went at a time when there were practically no restrictions. Since then, scuba diving has been banned (you can only snorkel) and they restrict the number of boats, the number of people in the water, and probably how long you can stay in the water. In spite of this, this manta cyclone is still one of the most spectacular and humbling marine experiences in my diving years.
When the mantas (and whalesharks) are not around, the diving in the area is not bad.
Lastly, should you opt to try this, it is important go during the proper moon phase. Check with the operators to see which - whether new moon, full moon or either one - is best for plankton buildup, which is what the mantas and whasharks are there to feast on. I went during new moon and it was good.
