As for scuba gear allowances....
Maybe 6 years ago, Frontier Airlines (before it was sold and turned into the very worst carrier on the planet) introduced a new policy regarding scuba gear. They announced it as a positive--for a mere $100, you could carry up to a specified amount of scuba gear, including a scuba tank and a speargun. I emphasized "up to" because the policy said that the existence of ANY scuba gear up to that amount would require paying that $100 fee. Have a snorkel mask in your luggage? That'll be $100, sucker. There was a real uproar on ScubaBoard, and in part because of that, people flooded Frontier with complaints. I was one of them, and I made a couple calls to people who clearly had no blessed idea why that was such a stupid policy. What is wrong, i would be asked, with charging that much money to enable people to bring scuba tanks and spearguns on a flight? I tried to explain that only a tiny, tiny, tiny portion of divers bring those items, and 99% of people bring gear you would not even know was there unless you opened the luggage and looked. They were adamant about it. Maybe you could get away with it, but if they saw you had a scuba mask in your luggage, you would have to pay $100.
Eventually I was put through to someone pretty high up who told me to relax. He was a scuba diver himself and understood the stupidity of the rule. Sane people in the company had communicated that message of idiocy effectively enough that the new rule had been rescinded within 24 hours of its announcement.
I suspect that more than a few airlines have such rules that they will simply not enforce. If your gear (with the exception of cylinders and certain batteries) is stowed away in your luggage, no one is going to say anything.