There is no problem running containers using KVM on an NFS. This works quite nicely. For VZ, these usually cannot not be on an NFS. For containers hosted off the NFS, you also need to take into consideration snapshots. There are some limitations when trying to when trying to make snapshots of running containers. That simply doesn't work.
If you want RAID1 mirror for redundancy, then place the entire proxmox installation on the server with the RAID1 array. If your single drive system goes down, even if you have a NFS with a redundant RAID type, it won't help--everything will be offline, even though you'd have your data on the redundant nfs drives, which is still better than loosing your data in a single drive system.