Hello,
1. It is automatically on detection of mobile devices.
2. No, you can’t control breakpoints because it based on detection of mobile devices independently of their screen size.
3. You may add the site to your home screen and then when you go on site you will see AMP version. Also you may add your site to home screen with advanced param in url https://yoursite.com?amp to prevent extra checking in code if it is device ready for AMP.
4. We didn’t test it for now but we pretty sure that you can use it. As you know it is extra plugin and anytime you will have any questions about it’s work you may just temporary disable it and write issues you have to us to fix but for that time use default version of web-site.
Regards