This functionality is now in the new editor. I highly recommend you switch to using the new editor, as the legacy editor will not be supported indefinitely.
Do you mean defining that a region in a template should be only replaced with a snippet? Or do you actually mean that the template should define which exact snippet in your Marketo...
The my.tokens can actually always be inserted in any email. The only thing that matters is if the campaign sending the email lives nested under some program/folder where that my.to...
You can add the lead in SFDC to a Marketo campaign, which will send a fixed email that lives in Marketo. That doesn't allow the sales person to edit any of the content in the email...
If you are using templates from our library, you should not experience this issue, as those templates are referencing locations outside of your design studio (namely, on http://tem...