I am going to call the commercial vehicle enforcement division and find out about the inactivity period and medical requirements. I'll post what I find out. I have a feeling it is going to be like you say- card required at all times, and after X months it reduces to a C.
Calif doesn't care or takes the time to find out if a CDL holder is active or not. I know this because I've forgotten to get my medical renewed a few years ago when I wasn't driving and got a notice. It basically said my CDL was invalid until I renewed my physical,but had a Class C.
Here iin Va. they do not even ask for your medical card when renewing your cdl. I know a person who hasnt had a physical in over 10 years but yet, he still holds a cdl.