I can confirm that the opennebula including the frontend runs fine in VMs. I have a 3 node HA cluster setup (RAFT) running like that. My master nodes run on vmware and manage a bunch of KVM hosts but the management instances can obviously run on the kvm nodes as well.
My ceph knowhow is as well limited as we are still working on getting it up and running but ceph needs some nodes and to run real workload on it you will want to use physical nodes that hold your osds. Vms for the monitors and so on are ok and for testing you can run the osds in vms too but if you really want to run serious load on ceph you will need hardware for it.
On the networking side you should be fine. I have some smaller node connected with only 1G and those host 5+ instances. It highly depends on you workload though. If the instances are very active you can run into issues fast. The instances produce network traffic and the datastore will do that as well so if your workload does not involve a lot of these tasks you will be fine but heavy IO instances will soon saturate your 1G uplink.
As my instance networks get their ips via dhcp and not opennebula i can’t offer any advice on your accessibility question. But basically if you can route the traffic to your instances you will be fine.