WS Case 50 is more on point. WS Case 123 is about when Starboard breaks RRS 14 while Port breaks RRS1 0, and Starboard's inability to be exonerated if there is damage.
Robert quotes Case 50's language referring to "a genuine and reasonable apprehension of collision" on the part of Starboard. Case 50 says that if S never changed course, P could not have broken RRS 10, as Michael says above. So that is the first question.
The heart of the problem are the questions, "What is reasonable?" and Reasonable to whom?"
I line up with John to say the jury is the judge of what is reasonable under the circumstances, which will involve questions about the type and maneuvering characters of the boats involved, relative speed and distance, and wind and sea conditions for a simple RRS 10 case. This translates into the time required for maneuvers and the space available to make them (see definition Room). These are the facts for which Michael is saying a good jury will look.
The point is to avoid making decisions (about Part 2 cases, anyway) based on semantic arguments whenever possible. That avoids over-empowering both nervous Nellies who bail out too early and parties who want to assert they are the final judge of what the rules require on the basis of their great experience or skill. 
Case facts say the closing range was 2 meters/second. If I were hearing this as 12 Metre case, I would not penalize P if S altered course at 80 meters. We are 40 seconds from contact at that point. However, I would keep it in mind how long it takes a 12 Meter to tack in judging how close is too close. A judge can benchmark that by watching the race and timing some tacks on the day, under existing conditions. We aren't too far from the point where P in a 12 M would be unable to complete its tack clear ahead without S needing to alter course. If it's an Etchells case, the protestor will find a very unsympathetic jury.

Satish' analysis looks most excellent to me.

He doesn't answer Q5 directly but effectively deals with it in his Q3 answer. If C felt they had ROW and that this somehow overrules rule 18, I suggest they read the preamble to Part A the last sentence of which says "However, some rules in Sections B, C and D limit the actions of a right-of-way boat." Rule 18 is one of those ;-) C is toast!

John, I am glad that the WASZP rules are working in practice.  I think that the wingfoils will go a different direction, with little or no limitations, at least in part because they are always in the standing position.  I think that should be up to the fleet decide and adopt in their class rules so we shall see as the class matures.

I'm with Phillip, in the definition its a measure, and that measure should be consistent. A less than competent crew is not entitled to more room than an expert one, and would be breaking a rule if they require it (lets leave room made freely available out of it for now).  Similarlty an expert crew is allowed the same room as a competent one, even though they may not need it,

John, I agree with you about reasonable apprehension and skill level, but I think its a red herring in the context of Room and Case 103.

The world sailing judges manual changes on improper act or ommission in each version, each seeming giving judges more room for interpretation. Just track the change next time, it may be what gordon and i may expect.
The old bad race management hidden behind the word may not shall,, may not be tolerated.
A time of change perhaps! I believe for the better if we are to improve standards and improve the sport.