Not that it matters, but I don't call it plan A until booked. Plan B to me is what I do if I have a confirmed booking a week in advance, and I get a call at 9AM on the day that she can't meet at 2PM after all. Sometimes I choose to believe her (probably untrue) excuse and book her later. This has worked out. But if this happens twice then end of. The plan B is a regular or a brothel that I can phone at the last minute. Or some times plan B is to not punt.
What you are talking is just the pain of booking. Sometimes I will ask email 2 or 3 girls at the same time asking if they are available in, say, 6 days time. I feel fine about this, as to me this is not a booking just a question. I would not ask if I didn't fancy a punt with them, so I am likely to book whoever responds first. If another one does respond later, I just tell her the truth, and say I'll hope to arrange a meet in the future. This could result in me being seen as a time waster, but I have not noticed a problem yet. Plenty of Fish. I guess this behaviour is driven by the TCM rule (see above) too.