Re: How to address a Shihan, Sempai, or Sensei?
In my time in the USAF, ASU, and independent aikido worlds, I've never heard any of the shihan addressed by anything other than 'Sensei.' I've also never been in a dojo that used 'sempai' as part of a form of address.
On the mat it's always polite to address the person teaching as 'sensei' although many dojos are more casual than that. Personally, I'd rather just be called Garth but there are a few people in my dojo who call me sensei and I think it's a waste of time to correct them.
In correspondence with senior Western teachers who I know, I will often address a letter or email with 'Dear <first name> Sensei.' For a more formal letter or with somebody I don't know, I always use their last name instead.
When I'm taking a class from somebody senior, I try to address them as sensei even if I'm on a first name basis with them off the mat. It's polite, shows respect, and preserves the mood of the class.
When in doubt, though, I always use 'sensei.' For example, I've been to many seminars with Ikeda Sensei over the years and been part of a bunch of lunches, dinners, and potlucks with him. He was at my house once when he was in Pittsburgh. He introduces himself to peoples' family as Hiroshi and he's generally very easy going. All that being said, he does maintain a bit of a Japanese reserve - we are not buddies so I always (and probably always will) call him sensei.
Anyway, that's been my approach on this issue and so far it's worked......