@Cams3
Just a small reminder:
Not knowing who this mysterious man is, Aria acted very cautious and used Mielle's name as a cover. After all, Aria's reputation was at the bottom of aristocrat society.
Aria figured that the Mysterious Man would be suspicious about how Aria (already) knew about the Gambling Den incident (thanks to being a time traveller), so she pinned the merits onto Mielle. That's why the attendant started delivering gifts to Mielle instead of Aria, which explains why the Mysterious Stranger is confused about Aria not receiving any gifts
The same scenario happened with the Fur Trade: Aria gave viable information for the Count's business venture (again, because of her being a time traveller). But this time, the Count gave the credit to Mielle, most likely because he favors her over Aria, and the fact that she is his legal wife's daughter. After all, Aria is the daughter of a concubine.
You might think it would be best for Aria to use her real name, but keep in mind, she doesn't even know this person. Secondly, if Aria was exposed to be the real informant - both on the Gambling Incident and the Fur Trade - her standing would actually rise by leaps and bounds, while Mielle would become the incompetent of the household. And the funny thing is: Aria did not even set this up for Mielle's folly.