Water line pressure slowly drops & well kicks in every 20 minutes. Replaced Check Valve Worked for 2 days & then same problem
Depending on conditions in your well or the care of your well contractor, the check valve might have gotten something (new, or old) caught in it. You could certainly try running the well flat out for a while to attempt to dislodge/flush that out. 

It sounds like your contractor couldn't find the issue and just threw parts at it. i.e. "replaced all the couplings" is "fixing what hardly ever breaks."


Yes, pipe would be full of water if the check valve at the pump
(submersible pump implied in this question) and pipe are good.
It's hard to say for sure. Low pressure ratings on a well check valve would be surprising, frankly. Some issue with sand or debris in the well sticking the valve open is my go-to guess, but it's a guess. A defective valve would normally not work for 2-3 days before being defective.
3: Every 20 minutes is annoying and a waste of power, but not the
dire situation that every minute or two is.