Yes!
If starting with markdown, you may use a tool such as pandoc to convert it to a PDF, or just leave it in raw markdown.
Sure, that probably falls under Framed by the Apocalypse: https://binary-star-games.itch.io/fbta
It’s an add-on to Valiant Horizon that includes 4 Ages to put into the past of your setting and guidance to help you do that, 3 classes per Age (so 12 total) that can either replace OR be used alongside the existing 12 classes, and a spread of new enemies for each Age. (I realized belatedly that I never actually put the description in so that’s fixed!)
These are all good questions! A lot of the answers to these are going to be because the game isn’t 100% complete. (The final edition is crowdfunding starting today!)