The Renoir we see for most of the game with the nice ass suit is a creation of the Paintress, correct?
Yes, Black suit Renoir, Black suit Alicia and Verso are creations of the paintress, her ideal versions of her own real family. I guess the last wonder I have in this regard is did Aline/The Paintress genuinely hate Alicia or not?
Who the fuck are “The Writers”? Did they start the fire that killed Verso?
There's a big implication (perhaps for future games) that there's some conflict between the creative arts in 19th century Paris. Between the painters, writers, dancers, musicians etc. At some point the Writers fooled Alicia into doing something which allowed them to burn the painter's manor down, and Verso gave his life to save his sister.
What’s the significance of the Gormage and why does the Paintress need to kill everyone of a certain age? Is it just to count the passage of time since Verso died?
Think of the monolith as The Paintress' Battery before she runs out of power to resist the real Renoir. The expeditioners were figuratively trying to destroy the clock for advancing time.