1. Can I upload Content in HTML but edit it in the UI?
Yes, you can upload the HTML in the WYSIWYG editor and make the required edits.
In Navigation, go to CONTENT, and select Email Content. On the Email Content index page, click New Content. In the New Content dialog box, select WYSIWYG.

Once in the editor, click ‘Source’ and paste the HTML as plain text, then click ‘Source’ again.

2. Can I select a custom footer for just one email?
Yes. Your default footer will be assigned automatically to all emails. In the content builders, you'll find a footer tab where you can select a custom footer for the specific email.
3. What is the difference between a Template and Content?
A template is the reusable shell of an email, whereas content is the specific email you are looking to send
For Example:
Template : Newsletter
Content : July 6th Newsletter