It's soft copper tubing. I have used it when replacing old pipes in two story building where access was extremely limited. I have seen SharkBite connectors used on soft copper but it has to be perfectly round... which is sometimes difficult. I'd be switching to hard copper before using SharkBites. Personally, I wouldn't be using SharkBites at all.
It looks like type K soft copper.
Generally it is used as the underground supply line from the municipal source at the street to the house.
In most cases it terminates at the valve and a harder copper is used on the house side of the main valve.
Push to connect fittings need a perfectly round pipe in order for the rubber seal inside of it to function as ...
It's a flow restrictor. They are usually installed in shower heads or other types of plumbing fixtures. It was probably left in a tool box and found it's way into the pipe or was intentionally not installed... I hated those things.
I would leave the copper attached to the CPVC, if taint broke don't fix it.
I would cut it down a little so your new valves are not so high, just below the drain pipe.
You can use compression, solder or push to connect valves for 1/2" copper.
I would add support for the pipes, put a piece of wood horizontal on the wall and use copper pipe standoff's or ...
Venting a flammable gas may be safe as long as there are no sources of ignition in the area and the gas has time to dissipate. Kind of like having a propane or LP gas cylinder filled once the cylinder is full they vent the gas between the pipe and fill valve. Outside away from ignition sources this is considered safe so I would expect the same to be true ...