All of that sounds like a lot of complicated work, if you are interested in trying to implement it I will let you know once my current changes are in svn trunk. I did encode the subject same as the body as I saw that property and figured it needed to be the same.
The display name encoding issue also sounds like non trivial work, if that is important for you perhaps you would be willing to implement it.
For me these are not high priorities. I'm sure OSCommerce and others already have support for it and someday we will too, but in the grand scheme of things the mojoPortal WebStore needs a lot of things much more important than display name support in email as does mojoPortal in general. I need to stay on the priorities that I see as more important.