As @KRyan told me, it seems like the duelist PrC is a little bit broken. But my party is currently lost in the middle of savages countries far from any civilization and it is very unlikely that I meet any PrC coach before a long time.
I am currently swashbuckler 5/Dragonmark heir 2/duelist 2/blade of orien 1, and I was planning to keep leveling as a duelist for some time. I think this build is not completely unbalanced compared to the builds of my teammates as they don’t put out as much optimization effort as I do. I chose to train the class knowing its flaws, but I considered it was a realistic evolution for a swashbuckler.
Nevertheless, some duelist levels like the 4th: “+2 on reflex saving throws” are really boring, so I am beginning to research additional content about this PrC like official alternative levels or widely acknowledged homebrews.
EDIT: I am specifically looking for optimizations of the duelist class, which from my point of view essentially sums up to “a character with style (like the swashbuckler) using a rapier and no armor and adding INT to AC“. What I am expecting is mostly alternative class levels, either as a few errata or a total redesign. I would rather it being some already well known content so that it would be easier to negotiate it with my DM.
(In case you were wondering, I did try negotiating taking only 3 levels of swashbuckler, but our DM insisted on us taking 5 level on our baseclass before multiclassing. Plus I liked the flamboyance of this type of character and did not want to change it to much. KRyan suggested me rebuilding as Factotum or Warblade but they are from books that are not allowed in my campaign.)