Re: Who to call "Sensei"?
As you can see the use of "Sensei" differs.....
Where I train, it is connected to rank. Anyone 4th Dan and above is called Sensei.
I've once visited a dojo where everyone who teaches is addressed as Sensei. I was invited to also teach a technique and was also addressed as Sensei. I felt really embarrassed as I was Shodan....
Sometimes when I teach beginners and show them what to improve by taking them for uke some of them say: oh, the master is training with me! then I reply: I'm not (yet) a master, just call me Simone, that's fine.
So: find out what's customary in you dojo. When visiting an other dojo find out what they do. And on courses I'd prefer to be rather too polite. As Bob said, people will correct you if they prefer it otherwise....
Don't worry to much and enjoy training!