In fact, Sokka is metaphorically the Avatar of the group.
What do I mean? Aang’s companions included one of every culture (well, bending culture: some seem to be more varied)… and Sokka. But he can’t get another Airbender. Hence, Sokka. He’s the neutral force between them. And he’s also the moral glue that holds the team together. Without his jokes, stupid and all, they’d be st each other’s throats in minutes.
“Ahh, Sokka! It’s Sparky Sparky Boom Man!”
It’s perfectly fine to admit liking it. I’ve heard it said that the show was written for adults and marketed to kids. Completely true.
In “The Siege of the North”, Admiral Zhao mentions in passing that Iroh had journeyed to the spirit world.
Yes, but that’s Iroh. Iroh is the Chuck Norris of the setting. 
More seriously, I believe you are correct. Other characters can and have interacted with spirits and the spirit world besides the Avatar. In fact, given that spirits can demonstrably manifest in the physical world, presumably anyone could do so if the spirit were so inclined. Interacting with spirits is part of the Avatar’s role, though.