GK - rule says any zip has to be fully zipped at all times - no flappage. obviously we do use a little bit of common sense when applying the rule and on hot days allow some unzipping, but not full length, but I tend to suggest to athletes - what you do out on the course where I can't see doesn't bother me, but if running around transition or for the finish then zip up. and for those at the pointy end, then they should understand the rules and comply and to be fair, we rarely penalise, just warn people. and more and more people are buying kit with rear zips to overcome the chance of a penalty.
it's a ITU rule now used by the BTF. and rules is rules. that's why all kit in events run under ITU rules has to be rear zips - no chance of infringing then.
wetsuits aren't compulsory between 14C and 22C - they're optional. if the water's around 20+ its easily comfortable to swim in without one. over 22C they should be banned under BTF rules. if the water is under 22C and you plan to wear a wetsuit you can wear anything underneath as long as it doesn't offer extra flotation.
as for trisuits or separates underneath - your call but a trisuit has the advantage of not needing to be changed for the bike and run, and will dry easily. yes - you can show nipples (swim in shorts, change in transition etc) but there is a rule that says that all front zipped tops must be fully zipped at all times on the bike and run. and no nudity in transition at all so if changing use a towel for cover. if buying a trisuit buy one with a rear facing zip as that way you have no chance of infringing the rule
trail shoes - only really needed if it's going to be muddy as they have better sole grip. if it's likely to remain dry I wouldn't worry and use ordinary running shoes.