Never heard of sunlight affecting a PCP to the point it would be dangerous. But you wouldn't "leave" any air rifle anywhere.
Yes you can overfill a PCP. They have a Safe working pressure on them which should not be exceeded. They are tested to a fair degree over that as a safety margin. Most operate best at a pressure below the SWP.
There's very little to learn Barry. A chat with the lads at the local club should give you confidence