I tried running it with Wine a few years ago and it almost worked. Perhaps Wine has improved since then, but I'm now firmly in the VS Code camp 

DocTypeCategory and DocTypeName are populated based on the contents of DocType. The DocType property can be changed even though the message is immutable.

TypeVersion is populated based on the value of the MSH:12 field in the message body. If you're attempting to modify the properties of an inbound message received from a business service, I don't think you'll be able to change TypeVersion with "Existing" set in the DTL editor because you can't modify MSH:12.

Are you working with messages newly arrived through a service that haven't undergone any prior transformations?

%Net.MailMessage has an IsHTML property; set that to a value of 1 and you can use HTML in your WriteLine() calls to format your text.