The PEX line should have a fitting that screws onto the nipple on water heater tank........so that the line (both hot and cold) can be disconnected from tank.
The check valve should be internal of the nipple screwed into tank.
Installing another shutoff valve is best and just remove the check valve internals doing away with it.
Turn a 2 valve WH system........
Into a 3 valve WH system.......
If cutting the PEX make sure cut is straight and no burrs.
Then use a Sharkbite (Home Depot), Gatorbite (Lowes) or even a Flair-It (Most any hardware) valve/fittings that just snap on.
