only tested when something didn't look right
I don't see any problem with that. If your tank is stable, it should be okay to test some parameters only once a month or less IMHO. I keep an eye on my alkalinity and test it fairy frequently since it fluctuates periodically, but I test other parameters such as Ca and Mg infrequently.
With that said, if you introduce a new product such as a new salt mix or a new additives, it would be a good idea to test your parameters. Some activated carbon are known to have some phosphate. So are calcium reactor media. When I switched my salt, I was surprised that the new salt had a very high alk level.
As for the regulators and CO2 cylinders, it's quite safe to use. I have not had any issue with them for the past 10 years. I have a #20 cylinder on my 75G planted tank and a #10 one for my 180 reef. A CO2 regulator is often fitted with an internal pressure relief valve. The welding shop that I get CO2 from always swaps out my cylinder with a pretested and certified Aluminum cylinder so that I don't have to worry about getting my cylinder tested and certified. Although I own my cylinder, I end up getting a different one every time I get CO2. The only issue that I have had with this method is that I sometimes get a steel cylinder, which is very heavy for me, when they don't have an aluminum one on hand. I am careful about carrying a CO2 cylinder in my car, though. I secure my cylinder well so that it won't roll around in the trunk of my car and would not leave it in a hot car for any extended period.
I use both calcium reactor and a small amount of kalk in my ATO. Some people run a calcium reactor and a kalk reactor together. This is to counteract the low pH that a Ca reactor tends to produce.
On the other hand, I am curious about automated two part dosing/Balling method which is dosed using peristatic dosing pumps.