Btw, that is 6 or 7 rules that came into play, in the span of 4 boat lengths, or roughly 90 feet, or roughly 16 to 17 feet per second, or about 6 seconds worth of elapsed time... ijs!
Yes, this is a complicated scenario. The relationship between the boats changes several times. That's why it's good to work out these encounters on shore ahead-of-time so you'll know what to do when the situation arises.
The pivotal rules in this instance are RRS 18.2(b), and the definition of mark-room. The boat that was clear astern when the other one reached the mark was obligated to give her mark-room. With the assumptions given, she did not.
I hope that helps,
Eric
