A simplified example:
— Do you think you can come?
— Yeah, unless (word here) I have a visitor.
"not very likely" seems like a solution, but is there any other shorter word/phrase to use?
One of my clients asked me if I was okay to meet with him on the coming Monday morning at 10am.
But I was expecting my mother's visit. She had told me she would visit me either on Monday or Tuesday, but most likely on Tuesday. So, this event was NOT unexpected, but rather what I was not quite sure of its possibility other than it’s low. And so words like “unexpectedly”, “possibly”, “by some miracle” don't fit.
A fuller example:
10am on Monday is fine with me, unless (with a low possibility) my mother visits me, in which case I would have to ask you to reschedule our meeting.