Thanks to the latest update by @AngelinaIgnatova (Introducing the new developer console and updated Managing your OAuth 2.0 (3LO) apps I can answer some of the questions:
#3 Do I have to create a stub Forge app for any other (including ACE) application to use OAuth2?
No, I don’t have to do it. Newly introduced Developer Console allows anyone to create either Forge app or OAuth 2.0 (3LO) integration. Use arrow close to Create app button.
Other questions remain unanswered.