I believe Convention Drive is a Town of Cary street and not private, so Cary Police should be able to issue valid traffic citations there. Another possible staff contact who could help is Doug McRainey in Parks and Rec; he was in charge of greenway planning when this one was built and has interest in safety issues related to greenways (and may know how to direct extra resources toward safety issues at greenways).
http://www.townofcary.org/Department...s/contacts.htm
How has the surrounding community responded to the greenway connection? Greenway connection retrofits in existing neighborhoods are often controversial. Supporters of this connection, including myself, hoped that it would provide your neighborhood with a more pleasant connection for walking and biking to the mall and surrounding destinations (compared to the busy 45mph arterial alternative). Detractors were worried about troublemakers using the connection to enter your neighborhood from the mall. How has it worked out?