1. Existing Peer relationship between our production ClusterA and our DR provider's ClusterB was configured using ClusterA's DEFAULT IPSpace. On the advice of NetApp support I created a separate (dedicated) IPSpace for ClusterA to talk to ClusterC. This means I had to create a new/dedicated IPSpace as well and add a route.
2. MTU sizes. My downfall here was that my Intercluster LIFs on ClusterA and associated Broadcast domain is set to 9000. However, I confirmed with our DR provider that the ports/broadcast domain at their end is set to 1500. So rather than heed all advice I've ever read/seen here I assumed that if mis-matched MTUs worked for the ClusterA-ClusterB peering it should work for ClusterA-ClusterC. So I only changed ClusterC's broadcast domain and ports to run at MTU 1500. NO GO. When I changed my new broadcast domain on ClusterA and associated ports to 1500 the peering worked.