OK .. I'll have a whack at it :-) (emphasis in rules added)

A1: a mark
A2: Because ....

  

A3: "left to Port" or "left to Starboard" (the "left to" puts the port and starboard relative to the boat).  
The language in Appx S7.2 is suggested as follows .. 

A4: Not if the meaning is applied in the context of def:sail the course, which requires the application/analysis of the taut-string of the boat's track after navigating the course.

The image from the other thread.  The mark below is "left to port" by the Yellow boat.

PS: And of course Happy Holidays to all my friends on RRoS.org .. wherever you are or whatever you do to celebrate this season!!! - Ang

My best guestimate .... have another drink.... it's almost Xmas ..... do not steer & sail while you are drunk ..... that will lead to a DSQ rightaway ! after the alcohol drug test after the race ......

1) the mark
2) mark is the nearest noun in the sentence. It would require a different construction to refer to the boat or anything else's side. 
3) it's conventional to say left to port if the mark is being rounded anti clockwise and vv. 
4) if I knew it wouldn't be unexpected [grin] 

Thanks for the replies guys & to Gjis for the seasonal alternative !
Q1. Two of you have said "the mark" and you may be right. It would be a lot clearer if the sub def made it crystal clear. Incidentally, as Angelo noted, the Def Sail the Course last line implies that it should be the required side of the boat.
Q2. Interesting answer Angelo. You have used J2.1 & Appx S7.2 to avoid having to avoid having to state the required side as required by the sub Def. However you also quote the Def Sail the Course which in (b)(1) complicates it again by stating "passing the mark on the required side", which again begs the question "required side of what ?". On Q2 Jim has stated what we normally do but has not stated what the required side is, as required by the Mark Room sub def (b).

If you are both right & the answer to "of what?" is the mark, then for a boat to "leave" it to port, it has to be stated that the required side is starboard, so that the boat passes the mark on the mark's starboard side (the boat's port side). Can you imagine the chaos if we state in the SI that the required side of the WM is starboard, when we expect the mark to be left to port ? Yet that is what the sub Def requires if the noun involved is the mark !

To make sense of the contradictions between the new Def Mark Room (b) & the Appx J & S provisions, sub Def (b) should be amended to : "to round or pass the mark on the required side of the boat, and". This would then make it crystal clear and the Def * the Appx J & S provisions would all work as we expect them to. Meanwhile, the solution as you have implied is not to state what the required side is at all, but to fudge it by using the words "to be left to port" etc instead !!

I suppose I haven't grasped something, but I don't see what the issue is. Maybe it's because personally I find the grammar sufficiently clear that I'm having trouble visualising it as 'the required side of the boat', but either way, as Angelo says, the SIs are mandated to tell us which way to sail the course. So, in my head, the path of room to be given is exactly the same whether I plumb in 'required side of the mark' or 'required side of the boat' into the definition. 

Robin we have "leave to port",  "leave to starboard", and now "leave astern".  Your discussion brings me back to the premise of my previous thread about "to leave astern". 

I think it confuses things that def:MR uses "to leave" when talking about "astern", simply because the other instances we use "to leave" is in a sail-the-course based thought process, and applying "to leave astern" in a sail-the-course way seems either nonsensical or at the least, trivial. 

To me it would have been more consistent with the rules and Cases to have MR culminate with "room to clear the mark", since that is the wording in def:racing. 

Robin ... re: Q4 .. I thought I'd play on your team for a moment and answer your Q4 with an example of where this might be confusing (or at a minimum, not be immediately clear) to racers, depending on how the SI's are written. I can use the Annapolis Harbour/Chesapeake Bay as an example.

Many of our races are not drop mark races but rather "out-and-back" where shipping channel marks (surrounded by plenty of navigable water) are used as rounding marks and the race basically finishes where it started.  Along with those marks are shoal marks which are surrounded by deep enough water for some, but not all boats.

Because boats may pass these shoal marks on the way out and then again on the way back, it is common for the SI's require boats to pass these shoal marks "on their channel side".

For instance.... this is typical here in Annapolis:  (emphasis added) 

So, now imagine 2 boats in different fleets (maybe there are 8+ starts .. and maybe a general recall or 2 tossed in for extra time).  The fastest-boat fleets are started first and remaining fleets started in descending order of avg speed-of-boat.  One boat is sailing "out" and the other "back" they meet, sailing in opposite directions, at one of these shoal marks.

Q5: Does RRS 18 apply between the boats, since both boats are required to SI pass the shoal mark on the same "channel side"?

I suggest that although both boats are required to pass on the channel side, in order to do so one must leave it to port and the other leave it to starboard. 

It's interesting. I'm  quite comfortable with 'pass the mark on the channel side' , but not at all comfortable with the construction 'leave the mark on the channel side'. Maybe because 'leave the mark' feels as if it's intrinsically relative to the boat, and 'pass the mark' relative to the mark. I wonder if it's relevant that most marks, being circular, don't have a port or starboard side? 

Clearer might be:
SI ##: ALL courses lie on the channel side of the following government marks:  Tolly point and Thomas Pt Light.

Faced with 'all courses lie on...'  in the SIs  I think I would consider it to be be an informational comment on where the courses would be laid, and it wouldn't occur to me that it defined the government marks as passing marks. 

SI ##: For ALL courses, pass on the channel side of the following government marks:  Tolly point and Thomas Pt Light

Jim ... I'd say you have got it correct.  This goes back to my sounding-board on this issue and that is "to leave" is done in the context of sail-the-course's taut-string.   

When examined that way, boats passing Tolly and Thomas Pt's heading south on the channel side, their taught string will leave the marks to starboard.  When heading North, they leave it to port (both Pts are on the western shoreline of the Chesapeake Bay). 

Therefore Rule 18 does not apply between 2 boats where one is on a leg heading south and the other on a leg heading north.  

Some SI's have spelled it out more clearly, which I think does a service to the racers.  Something like ... (not an exact quote). 

"On legs heading south, Tolly and Thomas Pt shall be left to starboard.  On legs heading north, they shall be left to port."