I would politely tell the SA that Hermes has been given enough time and that you are in the process of writing a formal complaint to corporate. And, honestly, if I were treated this way, I would stop shopping the brand.
In the first occasion, when the RTW was initially pinned, the tailor and the SA and the RTW manager could easily have come to the realization that this was not going to work. It doesn’t matter how many fittings, sometimes things are not cut for every person. My SA and tailor have shaken their heads at some types of items I like bc they cannot be tailored in a way that’s flattering. they’ve literally refused to entertain my idea of a runway jacket and silk separates and a backless dress, basically, saying regrettably no. ETA: at that time, they come to a strong estimate when the tailoring will be ready (which should been sufficient to determine whether it could have been done for a specific event)
And, then they should have guided you to something appropriate for Cannes. In the case of DHs MtM, (for a protégée ‘s destination wedding), he was inclined to go back to H bc he was thrilled with his MtM coats and jackets. He spoke with our H SA and mentioned he was also strongly considering the MtM Dior oblique suit. And, she was the one who pointed out that the Dior would best suit the occasion/ dress code. And she was right.
It’s not just about the sale; it’s about gaining the trust of the client AND demonstrating that the brand has the clients best interests. That is the way to keep clients coming back. JMO
ETA: all principled designer SAs (premier and otherwise) do this; my Chanel SA told me to buy an Hermes machine appliqué silk long skirt instead of the Chanel tulle bc it was best suited to fill a gap in my closet (she was right)
@Chrismin , my pants pic isn’t too flattering, but they look better on IRL lol. The tailor immediately started pinning as soon as I got them on.

They are the professionals, and they know what works, often before the client does.