"Do you have a lower bound?"

"Dwarves have dug down about a mile."

"...I will not singlehandedly eat a mile down from a good-size circle very quickly but if something disastrous happens when it does break through..."

Might be safer to leave it in your world with fewer unknowns, yes.

"Can you ask this Melian person? For reference the diameter of my planet when it's not riddled with holes is eight thousand miles ish."

The complication is that were she fully informed Melian might try to have you assassinated and I can't think how to get an answer without offering some information. We can try.

"I would rather not be assassinated. Why would she want to assassinate me?"

Wouldn't trust us not to use the magic against her if we had it. We've never warred with them, it'd be grounded in - general paranoia rather than specific grievances. I am happy to swear we would not employ magic in such a way.

"Possible I should hear out the general paranoia if there's a way to do that without being assassinated."

Humans aren't allowed into Doriath but they'll sometimes entertain conversation at the borders. I don't expect they'd assassinate someone who came to talk but I don't, ah, very strongly expect that.

"Is there a way to prevent that?"

We can send you with security but honestly if Melian's willing to start a war over it she could kill you anywhere within range of the border.

"...great."

I can send some Dwarves with the question, but we won't have a timely answer that way and may not get one at all. 

"And it'll have to go through translation." Sigh.

Destroying oaths only works if they are a thing and work as described, right?

"Uh, yes, I'm gambling here that if you wanted me dead you'd pick a different way to do it."

So after the experiment you'll have confirmation on that point, and then I can swear not to withhold relevant information and answer questions you might have about Doriath. I know what they'd say if they were willing to talk to you, it'll be approximately as informative as going there yourself.

"Okay, although that doesn't help with respect to the thickness of the planet. I'm not joking about this killing me if you're telling substantial falsehoods that I use to build a spell, by the way, that'd be a hell of a way to lose your only mage."

Doesn't help with the thickness of the planet, no. I didn't think you were kidding.

Cor sighs. He starts spreading out paper and sketching.

Elves come in and out and have conversation in Quenya or telepathically. Eventually he is told there are cows available.

"Won't need them till I have a design, but good to know."

Elves continue to work and speak and plan and have meetings around him. They all have long braided hair. 

"Dinner break?" he eventually wonders.