Sounds like someone is doing their race prep! Nice work!
For your questions regarding the water temperatures, I would prepare for both wetsuit legal and swimming without a wetsuit. I am not aware of a wetsuit only wave. If you chose to wear a wetsuit in the grey area, I believe you start with everyone else. That said, these processes continue to change as each race changes their swim start situations. So I would read the Athlete’s Guide, as well as ask at check in. Go to the mandatory race briefly and this is where you can get your questions answered and ask your questions. They have the most up-to-date information.
If the race is wetsuit optional, this becomes your decision whether to wear one or not. You will always be faster with one on. However you have to consider things like overheating, how much you have practiced in it and the time it takes you to get it off, etc.
As for latest rules on skinsuits, this is copied from the IMMD website.
If wetsuits are prohibited, what are the swimwear restrictions?
Swimwear (Skinsuits/Speedsuits) must be 100% textile material, which is defined as materials consisting of natural and/or synthetic, individual and non-consolidated yarns used to constitute a fabric by weaving, knitting and/or braiding. Simply put, this would generally refer to suits made only from nylon or Lycra that do not have any rubberized material such as polyurethane or neoprene. Swimwear must not cover the neck, extend past the shoulder, nor extend past the knees. Swimwear may contain a zipper. A race kit may be worn underneath swimwear.
This is typical preparation (and racing) for Ironman. You need to be as prepared as possible and then make the best decision for yourself. Keep planning!