Hi Kerry - If regulators are kept in the fridge it can affect the readings. Take a look at this thread on the subject - https://www.homebrewtalk.com/forum/threads/co2-regulator-in-fridge.12661/

The bolt on the back that says 1.8k is a burst disk sometimes called a rupture disk. It’s a safety feature. If the regulator is pressurized over 1800 psi the burst disk will open and release the pressure so no damage is done to the regulator.

Yes with an appropriate adapter. Most of the paintball co2 adapters have a 3/8th outlet not the the larger size like the 74 gram cartridges. Most of these mini regulators come with adapter plates for both sizes but I can't guarantee this one does the description and picture only reference the 74 gram.

The inside diameter for this regulator are 3/8. If the paintball threads are the same it will work.

Not unless you make your own adapter on the inlet side. the threads on a 74 gram CO2 are somewhere around 1/4 inch. The threads on a 20oz paintball are much much larger.

No just the 74 gram cartridges.

Never tried it. But it seems possible if you buy some fittings.