How do I get a view to display as themed within a simplenews html email?
This sounds very similar to what I did a while ago. I documented my setup on drupal.org. This seems to work for your already. I think your MailSystem is the source of error here. Make sure the MailSystem that is used by Simplenews is using the required style sheets and of course the custom templates from your web theme. Also note, that not all styles that are possible on a website an be used in an email, in fact, only a pretty small subset of styles will work at all.

Take a look at the Emogrifier module, it converts your stylesheets to inline styles for emails. Also, for images and other URLs, the Pathologic module may be of help as it converts relative URLs to absolute ones.