Can a Scheduled Job using mail_report send Reports as HTML in the e-mail body, rather than as a PDF attached to the e-mail?
Yes, you can send a report as printer-friendly HTML, in the body of a Scheduled Job e-mail, using mail_report. 

Go to Administer -> System Settings -> Scheduled Jobs. Edit the Scheduled Job parameters to add format=print, which overrides the default option, which is to send the report as a PDF file attached to the e-mail. 

See here for more information about managing Scheduled Jobs, including parameter options.