Re: Sensei? Sempai?
I tend to use sensei for anyone teaching the class. But we don't really have a set rule and nobody really seems to know what to do when a more junior member is filling in for our chief instructor.
Outside of the dojo, my sensei becomes "Pierre", despite his being a 6th dan. I also address at least one shihan by his first name outside of the dojo. But then modern Quebec is a pretty informal place, and out of the dojo, the local culture takes over and we're just a bunch of guys that have known each other for years.