For three months after Revelation he ignores his father's calls.

Yes. It should be possible to summon a demon and, depending how their powers work, either get a Silmaril from them or get the location of the patch of ground closest to the Silmaril from them or at worst do binary search over conjured models of parts of the planet to find the Silmarils. Yes, it should be possible to summon an angel to then dig the thing out of bedrock. And then they won't live life walking against the currents of fading, they will be whole again, they can summon some more for spaceship plans and head away from Earth much sooner, and much more powerful, than they imagined.

Also, the curious demon might go back to Hell and conjure some more Silmarils, if their powers happen to shake out that way, and might start handing out Silmarils to anyone on Earth who wants one, because why not, and the oath might still be in force, the risk is unimaginable - 

- they can of course get a Silmaril and then, if it turns out they can be conjured, not let the demon go home, that is merely incredibly rude and might strand someone a dimension away from their family for centuries. It at least does not risk unleashing the oath on this innocent world that has, at last, left the scars of the last time that happened beyond the reach of living memory.

For three months after Revelation he ignores his father's calls because he is childishly frightened, because his well-polished coping skills are fraying, because if he feels the tug of that oath on him again it will be too late to kill himself and therefore he wants, very badly, to do it now. For three months he wavers.

And then he answers the calls, and takes some vacation, and goes home for a Fëanorian planning session. He doesn't remember what loving them felt like but he remembers that he loved them, that it was once very important to him, and he knows he would do this for someone he loved. They plan and they read and they learn and they practice - without summoning, because daeva get the languages you speak when you summon them and so the first daeva they summon will know, if observant enough, they're not of this society - 

- he gets two weeks vacation a year, they plan very very slowly - 

- and six years after Revelation they have a binding and a few possible options for payment and a plan for the case where Silmarils turn out to be trivially conjurable and conjurable ones oath-relevant. Maedhros is terrified, and miserable, but no one can tell. He prides himself on that. 

Curufin doesn't want to do it because he speaks Khuzdul and the Dwarves who taught him it in confidence did not give him permission to share it. (They did give him permission, when it came up one optimistic night, to share it with his father should his father ever return to life, and so Fëanor speaks Khuzdul too.) Maglor's pretending to be a currently-dead pop star and that invites its own host of complications. 

Maedhros does not speak Khuzdul. He speaks the Black Speech in addition to a few human languages and Thindarin and Quenya, but the language won't scare daeva in itself (if they get his exact vocabulary, they might be frightened.) Maedhros picks a place in the castle in Canada that could be a room in an unusually wealthy human's house, unremarkable, and he painstakingly copies all but one bit of the circle they decided on together, and he calls in everyone to look and make sure he did it right, and he dismisses them all - one Elf alone is not obviously inhuman, two or more together raises suspicions -

- and he completes the circle.