"Elves cannot, it seems. I believe that neither can the Valar or Maiar. You are - bound to your own timelines. The first test subject for the process was an Elf, and it killed him." Leareth bows his head, his voice apologetic. 

Then the Valar were not lying about everything, he says sadly. They have told us that we are bound to the fates of Arda, while the Secondborn will have free will but be, consequently, bound to leave the world after a while. However, it was supposedly the same fact that made the Valar unable to be killed. Perhaps the Infinity Stones are...exceptions to the usual rules, somehow. Speaking of which, you said you would like to examine this one? He gestures to the Tesseract, which Tony left behind when he left Leareth and Fëanor alone.

The Tesseract, the cube-shaped container for the Space Stone, looks at first like a cube, but by turning it just right, one (who knows as much math as Leareth) can see that it's actually the projection of a higher-dimensional cube-analogue onto three-dimensional space. Directly visualizing the higher-dimensional space in which it really exists is impossible for human minds, or even most elvish ones, but by observing the twisting of the cube as he turns it Leareth will be able to see it obliquely. From just the right angle, he can see the tendrils that extend from the singularity of power inside the Tesseract to every point in space. This is...some kind of Master-Node—responsible, apparently, for the very concept of whereness itself, and its power is as vast as the universe.

Leareth is a lot better at mathematical visualizations than most people. He still can't quite wrap his head around it, but he has enough to work from, and the geometry of it doesn't make his head hurt even if actually using mage-sight does. He gets in a peek anyway, until the tightening band of pain around his forehead warns him that he's pushing too hard too soon. 

"Nayoki, can you...?" 

Nayoki is finding that trying to look at the Tesseract's geometry is making her head hurt, but she can try to get a closer look at those tendrils. 

Is there any obvious way to interact with them, using her magic, and follow one to a specific place? 

Touching the tendrils is a really bad idea; if she creates a path for energy to flow from the Stone into her it will very quickly be more than she can control. But she can see that the tendrils are all about the same "length" (if length is even a meaningful concept here), even those that attach to points very far from the Space Stone's projection in the surface world; it might be possible, with the Stone's help, to route a Gate through the hyperspace in which it exists. Possibly she could also set up a Gate and then attach the Stone to it without ever having power flow directly through her.

Nayoki conveys this to Leareth.

Leareth thinks in silence for a minute, then looks back to Feanor. “I can work with this. It will take some caution, to do it safely, but - two days’ work at most, I think. Once I am recovered sufficiently to do the mage-work required, that is. …I could likely do this both more safely, and with added convenience, by building a basic permanent threshold, but that would require longer. A week at least.”

Alright. I'll let the others know.

And he goes and relays the information to Tony and Thor.

Leareth lays back for a minute, closing his eyes. 

"...Nayoki?" he says, after a minute. 

"Mmm? Do you need anything?" 

"Can you reach the water jug for me? ...Also I wish to know if I can see Vanyel." 

"You do not look very ready to get out of bed!" 

"I will manage. I want to know exactly what happened." 

Sigh. "Savil told me. It was after they had retrieved the Time Stone, but - there was a local problem, a war, and Tony had context and...hoped they could end it. Which they did, but not without complications." 

She explains about the Ring. 

....Wow, Leareth is a surprising level of furious about that! 

"Tell Tony that I wish to speak with him again." 

"- Are you sure? Leareth, perhaps you ought rest more, before - addressing this." 

"Now." 

Sigh. 

:Tony, Leareth is upset with you about Vanyel's injuries on the recent mission and I think he would like to say this to you. ...If you are busy I can tell him that: 

Tony is in the middle of discussing what to do about the Reality Stone with Thor and Fëanor. Asgard is friendly territory, and the best bet is probably for Thor to go in alone, but they're all hesitant to send anyone anywhere without backup.

I am busy, but not uninterruptibly so, he tells Nayoki. Tell Leareth that I did what I thought was best, but I didn't have nearly sufficient information on your magic to make decisions involving Vanyel, and I'm sorry for acting rashly. If he would like to help inform me sufficiently so that something like this doesn't happen again, he is welcome to come find me. Perhaps he will have input on our current plans as well.

Tony is genuinely sorry about what happened to Vanyel, but he is definitely not (indeed, never is) in the mood for a lecture that doesn't actually help him avoid making the same mistakes.

Nayoki passes this on. 

"I would not have thought it would take very much context on Velgarth magic to think that it was a bad idea to bring a very powerful mage within range of the Ring and not warn him that it was mind-affecting! ...The part about the Gate was perhaps less foreseeable. Anyway, I am happy to explain our magic and its limitations and downsides to him in more detail." 

"Now?" 

Leareth sighs. "I suppose it is not that much of a rush. And I do need to rest." 

Nayoki passes all of this back to Tony.