Yes that is possible if you add an extra textbox to the email template edit page that displays the raw HTML code of the HtmlBody attribute.
I've added this textbox in an extra tab:
Of course this is possible. I usually create an extra tab page in the emailtemplate editor where I can view and edit raw html. Then you can just copy whatever html Outlook produces in there so you will send exactly the same thing that Outlook would produce.
If you want to dynamically insert images that takes a bit more work. The best way to do that is probably to inline the image as base64 encoded content in the email instead of as an attachment.