Answering A only:
a) Discourse works without issue on mobile/smaller screens. I’d highly encourage you to check out meta, or the testing site https://try.discourse.org/ to see how it looks. While there is an app, the only benefit is push notifications, and you have to be hosted (or customize the app) to make that work. When you “open a forum” via the app, all you’re doing is loading the the mobile site in your phone browser.