For example: Two concealed move markers are located within 4MPs of each other. One contains a hidden NVA company, while the other is a dummy counter. Could the NVA company move into the dummy counter without moving his original concealed counter into the same hex as the dummy counter?
Yes. (See 16.1 in latest, updated rules).
Additionally, is it now possible for the once dummy counter to continue moving with the reaming MPs of the NVA unit (in this case another 5 MPs)?
Yes. So long as neither the marker nor the unit exceeds its movement allowance. See 4.1.3. (Implied if not stated.)
If the above example is true I could see the PAVN player setting up a chain of concealed move markers without neccessarly having to move them, but still be able to move his forces around.
Indeed. A successful PAVN strategy...