I have to confess that I know zero about ISAF racing rules, but I do want to comment on one thing you mentioned.
In the situation you describe, power gives way to sail, period. So the powerboat must avoid both, and we can take it out of the equation.
That is not correct. When a sailboat is over taking a powerboat (even though this may not happen all that often) the power boat is the stand-on vessel and the sailboat is the give-way. I'm being picky, but also if the sailboat has the engine on (or perhaps in gear?) even if his sails are up (motor sailing) the sailboat is considered a powerboat and must follow those rules. Hope you don't mind my critique. The more I post here I'm sure you will get an opportunity to return the favor. :-)