A: it is a "hub of VPNs" where the remote points are connected to it using physical networks ("underlay") and establishing tunnels to build the private network ("overlay").
On the other hand, the Datacenter Edge connects to the internal network of a Datacenter which hosts services/applications, giving access from remote points to them through the overlay.
A: For small networks, up to 200 remote points and 300Mbps aggregated, is available a solution based on proprietary hardware, Teldat RXL14000.
For any size of network, software vRXL is available in ISO format, to run on bare-metal or on a VM on KVM, or AMI format (Amazon Machine Image) to run on Amazon.
A: Yes, it is possible to use 2 Datacenters Edges in high availability mode, in active/active configuration, i.e. in a usual situation each one is supporting X remote sites and in case of one of them goes down, the other one assumes its connections (2X connections).
A: Yes. A Datacenter Edge (or couple of them in redundant mode) provides services to a specific number of sites and supports a specific aggregated throughput, but as many Datacenter Edges (or pairs of them in redundant mode) as desired can be installed.