I don't know why with INV maxred.
You just go and show your license at the RVP gate and they will let you take mogas cannisters through as well. As long as you don't want to go onto the terminal side of things as long as you have a yellow jacket I haven't seen anyone have a problem. Just don't look into the waste paper bucket when you walk past it. They really don't think its funny those that remember that incident.
The Airport town bus is fine for Kirkwall, its the tour buses I was on about to scarbrae etc. I have only ever got them from Stomness though I think it left at 9am and got you back for 5 and you did 5 sites or something like that.