Keep clear definition:

"b) when the boats are overlapped, if the right-of-way boat can also change course in both directions without immediately making contact."

Going back to the original post: "J24, lateral separation between boats is 1 hull Length".

If the windward boat is 1 boat length abeam of the leeward overlapped boat, it seems to me that the leeward boat has plenty of room to change course in both directions without immediately making contact, right? How did the windward boat not keep clear?

This seems like an ideal example where Case 77 would apply. In the case, the give way boat didn't violate Rule 14 since it "was not reasonably possible for B to avoid contact with A".

Read this way:
When, at a boat’s starting signal,
1. a boat is over the line, or
2. she must comply…

Fair enough.

I'm happy to accept your view then. 

Its clear that the overwhelming opinion opposes John's proposal for whatever reason.

I think the tribe has spoken. 

Thomas ..
  Both Case 77 and Case 91 when taken together imply your answer I think.   Both pertain to the obligations of a keep clear boat when  the equipment of the ROW boat is out of its normal position. 

A boom swinging from the starboard to port side is a boom operating in its normal operating positions.   A KC boat sailing DDW so close along side a ROW boat that if the ROW boat flops her boom over, without ROW changing course, that the boom hits part of the KC boat .. the KC boat has not kept clear IMO.  

In other words, a ROW boat that has to prevent their boom from moving fully to the port side to avoid contact with a KC boat has executed "an avoiding action" IMO.  

I think this is affirmed in the negative when you read the cases I linked to above.  In those cases they discuss the obligations of a KC boat when equipment is or moves out of its normal location.  

IMO, unless there is a case that says otherwise, there should be nothing unexpected about a boat sailing DDW on port-tack flopping her main over such that she is sailing on starboard-tack. A KC should be keeping enough space between them to allow that.

Brad:  I don't think so.  We have established that the boat on the right in the diagram, boat T, is the 'inside' boat.  She is overlapped.  But she is not a right-of-way boat, as she is the windward boat in the overlap with C.  So she must keep clear under 11.  So she doesn't meet the requirements of 18.4  She also has no need to gybe at the mark to sail her proper course.

18.4 typically applies in a situation like the following, where boat A is inside, right-of-way (starboard tack while B is on Port), and overlapped.

Boat A must gybe at the mark to sail her proper course.  She can sail to her proper course, which is the one she would sail in the absence of the other boat in order to sail the course as quickly as possible, but she can sail no farther from the mark than needed to do so.  [See the definition of proper course]  So in this case, she may swing wide, then sail close to the mark as she gybes, and obey 18.4