Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
The 'Range' in the menu list item element sets which list items at the current level the settings should apply to.
Yes, you can build mega menus using Cohesion. Building mega menus requires advanced knowledge of jQuery in Acquia Cohesion and CSS.
Yes, this is possible by creating a custom element.
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.