Thanks for taking the time to thoroughly respond to my comments. I'm going to take another stab at this, which may just bury me deeper in a hole or help me to climb out of it.
The definition of mark-room lists the elements of MR. When all of these items have been satisfied, then the last sentence of 18.1 turns off Rule 18 (“Rule 18 no longer applies between boats…”). Tacking, 18.2(d), turns off 18.2( b) and (c), but not 18 as a whole.
IMO, Blue has been given the MR she is entitled to after she passes the mark by more than a BL and as soon as she reaches a close hauled course on port (prior to her tack) where she can start sailing her beat to windward (Blue #4 in diagram). IMO, the last sentence of 18.1 turns off 18 at that time.
Does this type of relationship apply to boats in the zone with regard to ‘Mark Room’?
No, 18 can apply between boats, but because of their actions inside the zone, the only remaining 18 rule that can convey MR is 18.2(a) and that won’t apply unless they are overlapped.
With these statements, you indicate that IYO Blue has turned off Rule 18 by virtue of coming to close hauled on port tack. Am I getting caught up in thinking that it is the tack to starboard that turns off Blue's entitlement to MR but does not turn off 18 as a whole, as you point out. This is the point (@3) that I think Yellow acquires MR as, I believe, she is overlapped inside of Blue.
Def Overlap, [These terms] apply to boats on opposite tacks only when rule 18 applies between them.
I contend that 18.2(a) does give Yellow MR. Yellow is not behind the line abeam from Blue's transom.
What rule do you feel applies after Blue tacks that uses the term “proper course”? We both agree it’s not 18, so what then?
My thinking around the topic of 'proper course' is that all boats have a proper course anywhere on the race course (after the start signal of course) and is the course chosen to be sailed in the absence of other competitors (good, bad or otherwise). It is the RRS that occasionally use the term, mostly not, to either allow you to sail your 'proper course', prohibit you from sailing your 'proper course' or oblige you to sail your 'proper course'. Maybe the term 'preferred course' would be more appropriate when referring to actions on the race course when talking about situations that involve rules that do not explicitly use the term in the rule governing the situation.
With that in mind, @ position 3 Yellow is confronted with the decision of whether to turn and sail her 'preferred course' which she believes is within her MR privileges or to sail beyond the mark and duck a starboard tack boat that is sailing some other 'preferred course' and hail "protest". Remember that these are Radio Controlled boats and all of this is happening (with any wind at all) in a matter of 2-4 seconds.
If we go back to applying RRS 15 and/or 16.1 to this situation and Yellow is at, say position 3.3 she would be pretty committed to making her turn around the mark, with or without MR, and at 2.9 not so much.
I still think Blue made a pretty "bone headed" maneuver in the zone of the leeward mark and Yellow is suffering for it.
Thanks for listening.