Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
Yes, this is possible by creating a custom element.
In mobile first, your styles will inherit upwards from the smallest media query, whereas in desktop first they will inherit down from the largest media query.
The breakpoint indicator shows you which breakpoint you're viewing your website at in the bottom left of the browser.
Go to Extend > Uninstall and search for Acquia Cohesion Breakpoint indicator.
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.