I downloaded and ran the windows binary, and I can report that the
thumbnails are no longer being generated (good!), and
as far as I can tell the hanging of the portaudio/midi library on exit
has stopped (good!).
There is a major issue though with the typesetting - Denemo is not
releasing its lock on the pdf output files that LilyPond
creates, which in the present code means that only two output runs are
possible for an invocation of Denemo. (This is because
instead of generating a new filename for every typeset I changed to
just two filenames, alternating them each typeset, in the
expectation that the evince widget would release its open file
descriptor on the old one when loading the new one. It seems this
is not happening though, which is bad in any case, as we will run out
of file descriptors).