He will go to it and if he's not immediately shot at or something he'll poke his head through and have a look!

Not shot at, no.

Inside the tree there is, in fact, a village. Houses are carved from the trunk and wooden bridges and ramps connect the various levels where people live and work. There are a few individuals here and there in small groups or alone. Their features are somewhat elvish, with pointed ears and faces, bright coloured hair—blue, green, canary yellow—and eyes, and very long hands ending in claws. Quite similar to Maester Seymour and his bodyguards, actually.

Must be Guadosalam.

He goes in! Does anybody conveniently decide to greet him or should he just shout hello and say he's a foreigner and curious to know more?

The guado peer at him curiously but do not say anything.

"… Hello?" he says. "Foreigner, just passing through, curious about this place?"

"Welcome to Guadosalam," the closest guado to him greets. "You're... not a summoner?"

"I'm afraid not?"

"Are you here for the Farplane, then?"

"– I didn't actually realize this was where the Farplane was," he says. "I'm from kinda far away."

"How far?"

"I– do not think there's a physical distance I can give you? Not from this world."

They all nod wisely as if they understand perfectly what he's saying. "If you're not here for the Farplane, what do you seek in Guadosalam?"

"I was just passing through and didn't – actually realize this was holding the Farplane, I was just wondering what was here. Which I guess has been answered."

They nod. "Well, be welcome to our city, and may your travels be safe and full of life."

"Thank you!"

He'll have a quick look around, see if there's anything of immediate interest keeping in mind he has no money and Kaede probably doesn't know he's alive.

There are houses! And stores! And an inn! And a palace! And a door into the tree trunk that supposedly leads to the Farplane!

Theo is curious about the palace… but decides he's gonna go find out more about how the Farplane works, if he can be like 'I'm a magical thing from really far away is there any chance anybody knows a lot about something really important that I should know'.

He got some sort of explanation before, he knows people sometimes visit loved ones? He's not sure if that's a requirement, though. (Maybe he'll get weird spam. He's not sure. Might as well try.)

He goes through the door in the tree trunk.

There is a short corridor that leads to a room with stairs that end in a watery portal on the wall.

… Bizarre.

And if he goes up to the portal, and – assuming nobody is around nor stops him nor does he see anything through it – pokes his arm through?

The portal tingles, but otherwise doesn't feel like anything.

He will go through, in that case.

Some more stairs and—

—he finds himself on a floating platform in the middle of gorgeous scenery, with pyreflies flying everywhere and unearthly light illuminating everything.

Oh that's weird it's almost like those bizarre steps in Sin except not.

Does anything happen if he just stands there?

Nope.

"Hello?"