Hi there, I'm a mechanical engineer with who spent a few years working and then went back for an MS in ID. Currently, I'm working primarily as an ME at an engineering consulting firm. I can only provide insight on a few of your questions:
1. I have no idea how hard it would be getting a job in transportation design. However, that seems like an easy choice, to me. If the only place you see yourself being happy is transportation... then go to CCS. If not, then it seems to me like a degree from CCS would be unnecessarily limiting your future career path.
3. As a MechE, you can get an interview with a 1 page resume. For ID, you'll need to show a few pages of work to get the interview and then another 20-40 pages of work to get the job. The school will definitely influence the portfolio, but your sense of style and project work will ultimately shine through.
5. I don't think going to a specific school will get you a job that requires expertise from both fields. The job does exist, but from my job search experience, it seems like only at higher levels where a small product design firm is looking for an experienced ME with some decent design chops. But they'll be mainly interested in the ME qualifications of taking products through to production. While the ID concept at the beginning of the product design process can be expensive, it's nothing compared to the engineering costs that go into producing the first parts and eventually turning it into a profitable product. Unfortunately, as and ME fresh out of school (who then studied ID for 3 years), you won't have any of that experience. My guess is that you'll be looking primarily for ID jobs with your shiny new portfolio.
6. I've heard/seen good things about NCSU and RIT, and nothing about Houston. I got my MSID from Philadelphia University. The program is new (I was the 2nd MSID class to graduate in 2014) and is improving by leaps and bounds every year. They've had a BS program in ID for a long time, and a bunch of other design oriented majors, so it is not at all a new field at the school. The faculty are absolutely great and can help point you in the right direction of wherever you want to go. There are a lot of corporate sponsored projects, which is cool because you get some recognizable names in your portfolio. It is, however, a very "hands off" approach, as is probably the case elsewhere. Graduate school is completely different than ME undergrad. You're not studying for the test to get a good grade to get a good GPA. You're working to hone the skills and interests that you'd like to pursue out of pure passion. There are, of course, set courses to take and assignments to do. But in contrast to engineering, if you're just doing the homework assignments, then you're not doing enough. PhilaU will give you a fine educational experience, but it is really up to you in the end to make it count.
Since it sounds like you've been accepted at some other really great (and highly regarded) schools, and you're ok with the financial outlook, then why not go to the "better" school?
Anyways, hope that helped. Best of luck!