hi Kay,
as a (part-time) bi male escort, i'm surprised more of the gents haven't replied, after all we are the experts for anal sex - most of our bookings involve us giving or receiving (or both) anal.
You asked some very good and structured questions and I think got some good answers. I would just say from my experience, anal does get easier after the first few times, I don't think it's about stretching but just being more confident and relaxed. I can remember, in the beginning (when I did it in my private life - not for work) it really hurt every time, my friend I was having sex with was quite big (8 inches) but not massive. Somehow I learned to relax, after that it never hurt with him and I really enjoyed it!
When I started working, again it started hurting, even with much smaller ones! Then again I managed to relax and it never hurts again - even really big ones! Now I actually like the challenge of taking a big cock, I feel very proud of myself afterwards! It doesn't really hurt either, just feel a bit "worn out" and feel aware of my anus (but not in pain).
So what I learned is that it's all about having confidence, being able to relax (so your ring doesn't tighten through nerves) and then it's possible to enjoy it. Also I have a friend, ex female SW, we have been occasionally having sex for like 15 years. She used to hate anal (never did with clients nor me), but now loves it and can orgasm through anal, often asks me to switch from her pussy to her ass.
About being clean, I always douche before receiving anal. With experience, you can know if you will be clean or dirty based on what you ate and how long since last ate, but to be sure I always douche - this also is part of me feeling confident (knowing that the client will be clean). Almost always girls I have had anal with have been clean, but clients very often are not - male bums seem to be dirtier than female ones, for some reason!
About condoms, always use plenty of lube. I never had any problem with splitting or slipping off. As others have said, cocks have to be harder to enter an ass than a vagina in the first place.
Good luck, and let us know how you get on!
henry