#1 There is no good reason not to buy a used 72 that has been properly maintained.
#2 Luxfer and Walter Kiddy both used the 6351 alloy, Luxfer stopped using that alloy in mid 1988 and Walter Kiddy used it until they went out of business in 1990. As long as the 6351 alloy tanks can pass the eddy current or microscopic inspection every year there is no reason they can not be used either.
#3 There is no good reason not to buy a lined tank that has been properly maintained.
#4 The eddy current test is only for tanks made from the 6351 alloy. There is no reason to do an eddy test on a Catalina tank of any age as they never used this alloy.
All of these so called rules you listed were arbitrarily made up by dive shop owners. I still dive a lined 72 cu/ft tank made in 1965 that I bought used in 1970. It passed hydro a couple of years ago without any problem. The age of an aluminum tank has NO bearing on its safety, only the 6351 alloy tanks have a potential problem but even the majority of these tanks are safe. I would not patronize any shop that made up BS rules like this.