Are France & Son pieces always marked?  

Since many Danish...
Since many Danish manufacturers used letter stamps to mark furniture components, I am not sure that the stamped letters would indicate a lot? Even though your pieces are unmarked, if you found them in a catalog that would be proof enough, due to the complexity of the pieces themselves. Not to mention the expense of the solid wood used in them.
I have a Finn Juhl Japan chair ottoman that is unmarked. Other France & Son ottomans I have seen have been marked, but mine has no marks at all, despite its details being the same as every other Japan chair ottoman I have seen. I don't really worry too much about it not being genuine.

One
of my Peter Hvidt & Orla Molgaard tables in unmarked, but unquestionably genuine.