Always happy if I can be of assistance.
Jeff Jewitt appears to stick to dewaxed all the way; partly because of what you mention (thanks, was only aware of that polyurethane varnish doesn’t go with waxed shellac), and partly for no mentioned reason.
Tom Fidgen has produced videos on French Polishing. I believe he too uses dewaxed shellac. He also has the virtue of not mystifying the technique.
Prof. Johan Knutsson has written a very interesting book on “The Craftsman’s Alternatives” (sadly not in English). He details on the use of lac (the resin behind shellac) and its use during the 18th century by the specific Lacquer Guild. It is perhaps noteworthy that shellac in French is gomme laque.
Derek Jones mentions in his book “French Polishing” (ISBN 978 1 86108 711 9) that dewaxing was invented in 1830, though he does not state anything on what type of shellac to use.
Richard Bitmead, in his 1910 book “French Polishing and Enamelling” (Project Gutenberg) gives two pieces of information: French polishing had been use in France before the technique became popular in [Victorian] Britain; and that the shellac should be dissolved in very concentrated alcohol. The latter, I think, was achieved by filtering through active coal, a process discovered in France during the 1790s.
Then R. Bitmead eventually gives the final clue: Waxed shellac was prone to impart discolouring of light coloured woods, as to why the Society of Arts in 1827 offered a gold medal or 30 guineas for a polish deprived of the colouring matter.
All in all of the above: Dewaxed shellac is to be preferred.
London, UK; Boston, MA