You can get back and forth between Harmondale and Eeofol once the portal in Eeofol has been opened, but IIRC there is no route to and from Nighon without going through one of the Connecting Tunnels. I don't remember any coach or boat.
I don't know about you, but after I'd solved the tunnels the first time, I really didn't want to have to do it again. It was easy to get lost, and it took a lot of time. The getting lost is solvable by learning it, but the time is takes is not going to decrease more than marginally.
You might enjoy the prospect, but I found that I really did need somebody with Town Portal. The second time I played the game, I cursed when I realised that I had failed to drink from the fountain after my first trip to Nighon, so that my visit had not registered, and I had to go through the tunnel a second time.