You guys are going to push, pull, or drag his car out of there with it in P?
Nah.
Only locks 2 tires, maybe one, depending on the differential.
I put my truck into 4wd, I have more traction than he does.
That being said, I'd simply tell him he's tresspassing and to move, and call the popo.
Especially as an employee. No skin off my nose.
No, it depends. At an 8 or 10-pump station on some rural stretch of freeway, when there aren't more than 2 or 3 pumps occupied, you're not likely to inconvenience anyone by going in for 5 minutes. Especially not if the gas is pumping half the time you're in there.
I'd be careful here, some people take this as an opportunity to have you 'donate' to their gas fund. Those pump hoses can stretch quite a ways.