Re: Sensei? Sempai?
I'm sure there are a million different answers to this. In my experience there are quite a few dojos in which whomever is teaching class is 'sensei.' That seems to be especially true if the dojo has a number of instructors of similar rank. Others, in which the head instructor is MUCH more senior to everybody else, they are 'sensei' and generally everybody else goes by their first names.
In my dojo I prefer people to just address me by my first name. A few call me sensei and that's fine - I don't see any point in making a big deal out of it one way or another.
I have heard (and used) a hybrid form off address - using the first name followed by 'sensei.' To me it sounds a bit more casual than just 'Sensei' but more formal and respectful than the first name alone. I use it with people who are my seniors and whom I know very well. If I'm ever unsure, I always default to 'Sensei' - better to be too polite than not polite enough, I think!
Anyway, that's my $0.02.
PS I think 'Jensei' is very cool.