Do I have to use IMCE?
No. Acquia Cohesion allows the site builder to select, either use of IMCE or Entity browser for browsing and selecting images.
Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
If you're removing styling from a base style, those styles still apply until you have saved the base style.
Base styles are CSS styles for HTML elements.
Yes, you can use Drupal fields in certain parts of Acquia Cohesion.
It depends on the use case. For fields and their field formatters use a 'Field' element, for data without field formatters, use a token.
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.