long: "then type the tempo of the incoming song as the tempo for the copy clip "
I think you mean type the new tempo as part of the name of Scene 2.
OK, that's very intriguing. The idea is to launch Scene 2, which has an unwarped clip containing the end of Track A, and a warped clip containing all of Track B. This would give me the crossfade I need, and the tempo change.
The tricky part here is that the Scene would have to be launched at exactly the right moment, and I think that would be tough to do. But I bet it could be made to work.
Anyway, I'm glad you mentioned this, because I bet a technique like this could come in handy in some other situation.
Yes, I do now have a simpler solution, but it's only because you suggested making the tempo change during the playback of Track A, instead of Track B. In retrospect it's so simple, but I wasn't able to think of it without your suggestion. So thanks again!