A1. There is no MS in CS at CMU, so you cannot shift to the CS department for a direct masters in CS(either ways program transfers are almost impossible).
However, i have some good news for you.
MS in ECE at CMU is a very flexible course and you can switch from DSP into whatever you decide to pursue, your course registrations are done in july or august and you'll be talking to your advisor after you reach there(either ways they wont mind your field switch, you choose the courses you want to take, they can only advise you).
Secondly, you can choose most of your courses from CIT(carnegie institute of technology under which the entire CS and ECE departments operate) which would count towards your degree.
A2. Students who're in the MS in ECE usually get pretty good internships and later even better jobs, so your $70K would typically be returned within an year of working. Do mail a couple of current students who do work closely relating to what you want to pursue and ask them about their current job prospects so that you can get a clearer idea.
p.s: i regret not applying to the MS in ECE at CMU because i thought i was a sure-shot into one of the other programs CMU offers