Hello,
Answers to your questions are as follows:
1). Yes, in order to configure a global cluster, each cluster within the global cluster must be connected with a cluster heartbeat. During the configuration of the global cluster, you'll configure the WAC resource (Wide Area Connector). That configuration will list each cluster in the global cluster and its associated IP address. The WAC resource will take care of the ICMP heartbeat between clusters.
2). The prior response is correct, the SRDF agent is included in the SFHA/DR version of the software. All replication agents are included.
3). Technically, you can create a global cluster without configuring a replication agent but in doing so you will be removing a large part of what the cluster is supposed to do. The SRDF resource in the cluster is supposed to interface with the SYMCLI on each node and take care of replication role reversal to each side of the cluster, i.e. demoting the primary R1(read/write) to R2 (read only) and the R2 copy on the DR side (read only) to R1 (read/write) and vice-versa on failback. It will also take care of syncing the sides up prior to/post migration. Without this resource configured, replication administration will be all manual and failover would need to be closely monitored to be successful. Resources would fail to come online on the DR side while the disk on that side is marked read only, and synchronization would be tricky.
Hope this helps!
J