"but in direct speech might well be accorded respect by the use of "Sir", or "Sieur" if you wanted to make it more French (but not "Sir [first name], which is a different title altogether)."
I'm not sure if I'm understanding you correctly. Are you saying, if the Duc's son (who has no titles) is first name Andre you would not address him as Sieur Andre? But if his last name is Benoit you would call him, Sieur Benoit?
No.
In England and Wales, monarchs could and can create knights which (now at any rate) are titles for life only, and since the 1600s there are also baronetages, which are hereditary titles. If David Smith is knighted or is granted the dignity of baronet, he becomes Sir David Smith, and is addressed as Sir David,
never as Sir Smith. (His wife, Dorothy, is Lady Smith, never Lady Dorothy, nor Lady Dorothy Smith.)
As far as I'm aware, although the French had chevalier as their equivalent title of knight and a holder might be called Chevalier Andre Benoit, he would be addressed simply as "Chevalier". That is, I might say "Here comes Chevalier Benoit" but in talking to him I would say "Well, Chevalier, what do you think of all these titles?"
A nobleman was
not automatically a knight in England or France, and I doubt it was the case anywhere else, either, nor could fathers confer the dignity on sons here or in France.
In English, "Sir" has been used as a general mark of respect for those without specific titles since the C14th (but
not "Sir Robert" unless that person was a knight or baronet, as above). I'm no expert on the French nobility, but I'm quite prepared to believe they used the equivalent of "Sieur" in the same way for those who were gentlemen of noble lineage but with no actual titles, just as a matter of respect and courtesy. However, I'm pretty sure they would
not use either "Sieur Andre" or "Sieur Benoit" when speaking of or addressing Andre Benoit, but it would just be "Well, Sieur, what do you think?". NB: "Sieur" properly translates as "lord" ("monsieur" as a term of respect for any French man is literally derived from "my lord") but it certainly doesn't have the same connotation as "lord" does over here. Here you would only address the nobility as "my lord" -- a duke, by the by, would nowadays at least be properly "your grace".
If you're writing fantasy which is only loosely based on medieval Europe, then you can do what you want. In fact in my French-based fantasy I use "Sieur" in exactly the way I've said the real French didn't, with Sieur Roparzh de Pichon being addressed as "Sieur Roparzh". However, if you want your work to be close to the real medieval France, you really need to get to grips with life at that time and place in great detail, and with all due respect, I don't think at present you've done nearly enough research to make it sound convincing. So if you want to give your work a semblance of authenticity, I'd echo Brian's advice and suggest you do a lot of reading around the era so that horrors such as "dukette" are not even contemplated in future.