a) If you leave the implementation for the GCM there and work as suggested in OneAPK docuemntation, the original API should continue working just fine in environments it was working previously. The Nokia Notification API does not work with other than Nokia X devices, thus it can not be used there though.
b) It is required, the GCM doe not work on Nokia X devices, thus in order to get the notifications functionality, you would be required to use the API which is supported, i.e. the Nokia Notifications API.
Do check the documentations provided in nokia developer site for tasks needed for the porting effords.