Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
Yes, you can build mega menus using Cohesion. Building mega menus requires advanced knowledge of jQuery in Acquia Cohesion and CSS.
Yes, the generated templates can be saved to the database.
The Youtube video embed is responsive so it should expand to the width of its parent. If you do need to set a width you can apply the Width CSS property through the Style Builder.
Custom styles are CSS styles for classes.