Last year I had to take a very large concert grand in to a choir loft with stairs just like you are describing. Two men from the choir helped me and we could not use the dolly because of the twists in the stair case. It was very scary and I’m not sure that I would do it again. I know someone who lost a harp having it dropped in exactly this way. Then again this is my job. Hard to say. A clause is a good idea.
In response to your “stairs clause” question, I definitely do! I don’t like asking congregation members, or members of the wedding for help and so whenever I take a wedding or gig I ask if the location is handicap accessible.
In my contract I specify this as well, in addition to stating additional cartage fee for any venue requiring x number of consecutive steps.
Make sure you stretch before and after! Nothing is worse than accidentally pulling something right before you need to play.