In Case 29, although the outcome is correct, the explanation wrongly
applies 19.2.c's continuing obstruction "lockout" to the situation's
19.2.b fluid situation.
19.2.c (Continuing Obstruction)
if "at the moment the overlap begins, there is not room for [a boat
clear astern] to pass between [a windward and leeward boat],
she is not entitled to room...
and while... remain overlapped, she shall keep clear."
This is a lock-out that continues during the overlap.
Conversely, there is no lock-out in RRS 19.2.b
(non-Continuing Obstruction)
"When boats are overlapped, the outside boat shall give
the inside boat room between her and the obstruction, unless
she has been unable to do so from the time the overlap began."
"unable... from the time the overlap began" is a continuing test which
means that she must give room if and when she becomes able to at
some point during the overlap.
In Case 29, although the outcome is correct, the explanation turns
the wording around and wrongly applies 19.2.c's lockout to the
situation's 19.2.b fluid situation:
"The boat clear astern may sail between the two overlapped boats
and be entitled to room from the windward boat to pass between
her and the leeward boat, provided that the windward boat
has been able to give that room from the time the overlap began."
A correct explanation would be:
"The boat clear astern may sail between the two overlapped boats
and be entitled to room from the windward boat to pass between
her and the leeward boat, provided that the windward boat is able
to give that room."
If 19.2.b intended the same lockout as 19.2.c, it would have been
written in the same way. It was not.
I think you are missing the essential difference between the exception conditions in the two rules.
Rule 19.2(b) ' ... she has been unable to [give room to pass] from the time the overlap began'
Rule 19.2(c) '... there is not room for her to pass ... at the moment the overlap begins.
The first refers to the ability of the outside boat to manoeuvre so as to give room, even if there was insufficient space at the instant the overlap began
The second refers only to the space between the outside boat and the obstruction, at the moment the overlap begins.
I suggest you read RYA Appeal 2014-4 for an explanation of the 'freezing' or 'snapshot' approach to applying rule 19.2(c).
RYA Case RYA2014-04
I agree with your logic but what you quote a the “explanation” only appears in the abstract of the case.
The statement in the full case 29 is:
“It [rule 19.2(b)] required W to give M room between her and the obstruction, unless she was unable to do so from the time the overlap began.”