Returning to start?

I have been doing some research on a question put to us at a rules-evening last Thursday. The evening was for members of a team involved in team racing. So the rules applicable are the Racing Rules of sailing and Appendix D.
I also want to look at this under normal Fleet racing rules (RRS) and under Match racing appendix C.

Here's the situation. Two opposite team boats have been engaged before the start and end up some distance above the starting line. Yellow changes course and returns toward the line. Blue does the same and gains a windward overlap of a about quarter boat length.
When the boats are at a distance of three or four boat lengths from the pin-end starting mark, the signal goes. Yellow at full speed wants to take the pin end mark efficiently and doesn't want to leave room for Blue on the inside.


The questions:
Does rule 18 apply and is Blue entitled to room or not?
If Yellow doesn't round the pin-end, but instead chooses to go for the line between the starting marks, is she entitled to room and has Blue to keep clear?
When does rule 20.1 come into play?
Does any of the answers change when only Fleet racing rules or Match racing appendix C apply?

I'll keep working on this and give you my view in a weeks time. In the mean time, you can do the same and leave a comment, OK?