Just as a follow-up, I ran this situation (without all the hypotheticals) by an IJ who happens to be a good friend and mentor of mine.

The IJ's take is that the redress request would be valid, but that the decision of whether or not to actually grant redress would come down to the jury's judgement on whether Boat B really has a case that the situation was no fault of her own. The IJ said he personally would not likely grant the redress if he heard the protest (the score would remain as DNF). However, Boat B has the option to request a reopening of the hearing.

Also, as Eric mentioned, The IJ poitned out that the PC cannot protest Boat B for hitting the mark, so she can't be scored DSQ.

My friend is an IJ for lots of good reasons, not the least of which is, he said: "The RC should be admonished for their indiscretion, and more importantly it should be done in an instructive/constructive way so they learn from their mistake." I think I was trying to make this point rather subtly, but certainly not as eloquently...

Mike