Hi there,
I'm not sure if you are still checking this, but I can tell you some things. I'm a current MTS student at Duke...so...hi! (I was looking for something else on Google and saw your post). On the one hand, there are plenty of MDiv people who are either sure they are not pursuing ordination, or who are on the fence. Trust me when I say that vacillating on that issue is totally normal. Now, probably most people in Duke's MTS program are at least intending on pursuing doctorates, but BY FAR not all. I am not. (Right now I am not sure about work in a parish setting vs. teaching in private secondary school myself.) Now, my impression is that it is actually harder to transfer from MDiv to MTS than vice versa - the MTS program is very small and selective. I don't remember if you can apply after your first semester to switch programs, or if you have to wait till the end of the first year, but either way you will have to wait for some grades and get a member of the faculty who knows your work well (ie, not someone you had in a big lecture class) to recommend your transfer. I would say, don't freak out about your degree program right yet - either way you will be in pretty much the same classes your first year. Either way, you are about to begin the most amazing adventure of your life...you will be changed.
Also, Reinhard Huetter runs the MTS program - he is the one to ask about these things. He is very wise.
Peace!