1. I have gone twice (2010 and 2018) timing it with the full moon of July, 3-4 days before and two days after the full moon, because full moon seems to be when the spawn of bonitos and/or snappers occurs. I had very good encounters on both trips. After my last trip in 2018, I concluded that there was definitely a decline in numbers from 2010 to 2018. Also, that it wasn't worth it to be there after the full moon because most of the whalesharks seemed to have left, and there must have been 15 people chasing after one whaleshark, with boats sometimes desperately charging ahead to place their guests ahead of the animal. Whereas, before the full moon, pretty much every boat and its snorkelers had its fill of whalesharks without crossing into the path of snorkelers from other boats. On good days, there is no doubt that you will see the entire Mexican armada of boats out there - so much so that from a distance, it almost looked like there was an island out there. But I would not let that discourage you because to me, it is the best and easiest venue to get to from the US for whalesharks, with blue waters (not green like in Holbox or La Paz) albeit not great viz due to the amount of fish eggs in the water. t
2. When I went, a wetsuit was accepted in lieu of a life jacket. On the really busy days, there were park officials out there enforcing the rules.
3. If it is whalesharks that you want, as I said above, this is the best you can do on this hemisphere. After Isla Mujeres, I felt that if I didn't see another whaleshark again, I wouldn't feel terribly disappointed. I feel that that my 2 trips there were very much worth my while and money, but I did not do the normal tourist thing. Tourist boats will take you and up to 9 others out at 8:30 - 9:00am, get out there by around 10:00, wrap things up by around 11:30am or earlier, so as to motor back and anchor the boat on Playa Norte around noon to serve you ceviche and sandwiches and allow you to swim or snorkel there until around 1:00pm, at which time they return you to the dock. Do the arithmetic and you quickly realize that your total in-water time with 2 snorkelers per boat at a time in the water, and let's say 10 minutes per turn, is peanuts.
I chartered for 6 days with a group of 6 the first time and a group of 3 the second time. I negotiated with the operator to go out at 7:00am, and head back no earlier than 1:00pm or later (depending on how the action was). Lunch was eaten out there at our chosen time, like in between turns or when we wanted or needed a break. I would do a minimum of 3 days because you never know when a day could be canceled due to a storm or turns out to be a slow day. The more days out there, the more you will see. For example, we had two separate encounters with sailfish one day. One day we had a lot of mantas in addition to whalesharks, including a choo-choo train of 5 mantas. We also saw dolphins out there. Of course, I am not going to say that these are everyday occurrences, but you won't see anything if you are not out there.
4. No idea - depends on how many available days and how much money you have.
5. In my opinion, this is as good as it gets, but it is all snorkeling. Not clear anymore if freediving is allowed. That's another thing that those park officials are on the lookout for. If you want to do it with scuba, the only places I know of that are exclusively for whalesharks are in Indonesia and the Philippines. As best as I know, all of the whaleshark venues in Mexico are snorkeling only. Placencia in Belize and Utila in Honduras are other known places that people go to specifically for whaleshark sightings in this hemisphere, and where you might scuba with them, but I don't know how much of a sure thing they are there. Galapagos, Maldives, Mozambique, Djibouti, and St, Helena are the other places where there are consistent sightings in season.
6. No. These ones are 20-30 ft max. The buses are in the Galapagos. I have seen 40-footers in Cocos and Socorro.