I'm going to guess that the difference is that Tailscale lets your machines find each other within a managed flat virtual network where as Iroh lets your applications talk to each other without any regard to which machine anything is running on.
Not sure about tailscale coordination server but once you establish connection to a headscale server, the clients don't strictly need headscale after that (although it's recommended to keep it active). So, maybe the only difference is headscale acts as a relay for once
Headscale is just a open source implementation of the Tailscale coordination server.
The coordination server just provides the IPs by which you use wireguard to connect. It can see that metadata (what machines are in a tailnet), but not anything else.
