For future reference, just wanted to update this thread to note that if you click on the email subject line in MSI, it opens a preview window where the tokens DO resolve correctly (both subject line and body).
I don't think I had noticed it before.
It's still a poor UX to not have it render in the tab itself but at least reps can click the preview and see the entire email.
Email tab in MSI
Email tab in MSI, showing ugly subject lines with raw tokens.
Click on subject and preview looks nice
Clicking subject line opens preview, tokens render nicely.
Underlying email fully tokenized
View of same email in edit mode, showing it is fully tokenized.