Obviously the ideal way of storing the boat is on a trolley ashore, but for singlehanded sailing this can be a pain. I dry sail using the car to recover the boat but presumably this is not an option for you? I would still suggest that if there are going to periods of a few weeks when you know that you won’t be sailing, you should try to get the boat out of the water.
If the boat is painted rather than varnished it should stand up a bit better. Frost is, in my experience, the biggest enemy, and really shows up any poor workmanship!
PVC covers don’t breathe at all, but it ought to be possible to arrange for some sort of ventilation by having an opening fore and aft. I don’t know what the weather is like at your location, but I wouldn’t use a cover on a moored boat due to the increased windage. I would just leave the boat open and visit her at least once a week to bail. Or perhaps fit a wave-action bilge pump.
I used to have a composite boat and the amount of work required to keep the varnished deck in good condition eventually made me switch to a World.