When your (working) cluster had 5 Nodes, 2 can fail (from the corosync perspective). So you can remove one without a problem.
From the ceph perspective it depends, how many mons you have and how your pool is configured.
The 5th node ist already...
As long as you have a L3 connection between your pbs and dedicatedserver it should work. You could portforward the source ip of the dedicted server to your pbs, or could use tailscale or a similar solution. The first way is probably faster, the...
May I ask why you don’t want to remove the node and re-add it? Probably it is the easiest way, and if the rest of the cluster and Ceph are configured correctly, you should not experience any downtime in your production environment.