Hi,
1. If the concept of Clean Page is a Canvas Page (blank page without any header or other stuff), than you have to create your own Page Template. Yes, you can use Widgets or Plain HTML by using Full Width Widgetized Template.
2. Contango Text widget is designed for Full Width Widgetized Template. You can use Standard WordPress Text widget in the footer.
3,4. Please share your blog url, So i will be able to share CSS Snippet with you.
5. It will need to change JS file. I will forward it to the Dev team, they might be able to share a JS code.
6. You can add “#” sign in the Custom Menu Custom Links URL, It will prevent the link to open.
Hope it will help you.
Thanks