Re: Who to call "Sensei"?
I had a funny experience in Waitrose (a local supermarket) a while ago. I guessed that one of the cashiers was Japanese from her appearance and accent, and when she returned my credit card to me I held both hands out, palms up, as if someone had passed me a weapon, and bowed my head slightly. She automatically said something in Japanese in response, which ended in "sensei". It felt very natural at the time, but by the time I got back to the car I started to see the funny side of it.
When I was in Tokyo a few years ago, the stallholders under Shinjuku station were all calling out to potential customers as "sensei"; and when I used to go to a Japanese osteopath I always addressed him the same way. I understand that in the context of Japanese culture it is used as a general expression of respect for someone's experience and seniority.
My feeling is that, in the dojo, there are no fixed rules for whom to call "sensei": from my experience I think I know when it is right to do so. Normally I address whoever is teaching the class as "sensei", even if I would use their first name to their face outside. Outside the dojo it feels natural to hail Japanese teachers as "sensei", but not non-Japanese.