Update-
Hopefully the MSI/SFDC editor will be improved in the future; at this point it looks like the editor is only build to make very minor edits to already-created email templates. After spending some time with support this is the response I got:
"The published Sales Insight email is nothing more than a HTML email template that is brought over from Marketo. In Salesforce the editable sections are ignored and you can edit everything in the email template. The WYSIWYG editor used in the Sales Insight App is borrowed from components of Salesforce and we do not have the ability to control the editor behavior that you are seeing. You can however use the WYSIWYG editor to adjust the text font, size, bold, italic, etc.
It is therefore suggested to type out your message and then highlight the text and use the provided buttons in the editor to change the font, grow or shrink the text, and back space out in extra spaces, etc."