Since the phrase doesn't give much detail, Des Forges could be anyone. It might be an author using a fictional name, or it could be a real person from history, literature, or even a local legend. Without further context, we can't really identify this 'Des Forges' precisely.
Hard to say. Without more information about 'Des', it's difficult to determine if it's a true story. Sometimes stories are inspired by real life but have fictional elements added.
Yes, in many ways it is. The manhwa generally follows the main plotlines of the light novel. However, like most adaptations, there are some minor differences in pacing and certain details might be emphasized or left out for the sake of visual storytelling in the manhwa.
I don't know for sure. It's possible that there's a kernel of truth in it, or it could be completely made up. Without more information, it's difficult to determine.
The name 'Nuit d'Issey' likely refers to a night-time concept related to Issey Miyake. The parfum might be designed to evoke the mystery and allure of the night. It could be inspired by the idea of a special evening or a nocturnal encounter.