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re: I ran all of the test cases on an Explorer and found that the results were
    all consistent with proposal

It's highly probably that all "spiritual descendents" of MacLisp have
this behaviour, because they all probably copied the optimization that
treats ",."  ",@" and ".," the same when found at the penultimate 
position of a list.  ["backquote" evolved during the mid-1970's at
MIT in the AI and LCS labs].

Steele's book was the first to formalize the meaning of backquote, with
his meta-language on p.350.  From this discussion, it is clear that
the step
   (APPEND ... (APPEND X NIL))  ==>  (APPEND ... X)
is arbitrary, rather than being an inevitable part of the intended
meaning.  Steele implies that either doing this optimization or not
doing it is fine.  But the Issue APPEND-DOTTED now seems to make
this an invalid optimization.  

My feeling is that we either have to go for the discriminatory version
of BACKQUOTE-COMMA-ATSIGN-DOT-COMMA -- the one that says that ".," and
",@" really don't produce the same code -- or else we have to go back
to the prior state for APPEND.

-- JonL --