1. It is not required to have a VR appliance per datacenter. If you are talking about physical datacenters, you may want a VR appliance at each site if you are replicating VMs in both directions
2. No. The VR appliance only needs a single interface for management and replication traffic. In version 6.0 and higher you can configure a second interface to carry replication traffic if you want mgmt and replication separate.
3. It depends on the version, from version 5.5 (I think) on it requires the web client