Why are you putting those two nodes in the same cluster as those before? Why not just for starters, create them in separate clusters and work with that?
Why you have external help? When External Audit comes(Grant Thornton,KPMG etc etc) first they ask you is "what if you get hit by a bus", who will support that and that. That is why you need to have external maintenance contracts.
You mention failover, and in that regard, it makes sense to use Proxmox <> Proxmox pve-zsync replication. Both proxmoxes have zfs storage for vm and cts and you implement it on one machine. Failove then has moving .conf files and starting the...
1. Yes it is because you don't have shared storage for all your nodes, which is usually ceph.
2. There are some aftermarket scripts to replicate whole pool, but you usually want to replicate different machines with different schedules, eg dbs in...
My recommendation is to use physical links to corosync, not vlan ones. As for the other things this is okay, all links should be active-passive in proxmox, so that you don't care if something dies ,and this is it. CEPH will work great in that...