() David Kastrup <dak@gnu.org>
() Sat, 20 Dec 2014 15:19:18 +0100

   I am missing the big picture here in some manner.  Does
   decoding not start from a byte stream but rather from an
   emacs-utf-8 encoded version of a byte stream?  That does not
   seem to make sense to me.

Have you tried specifying ‘:coding-type raw-text’?  I see in
src/coding.c line 5339 (from a mid-October pre-Git tree) the
nice number 1: