Leareth has been thinking the same thing. Most likely, Melkor thinks that he can build his own strength even faster - that time will increase the differential rather than decrease it. In which case, all they can really do is try to surprise him. 

The next day, he has everything prepared for the Gate – pieces of the giant threshold to be carried through the small terminus here, fully powered artifacts for shielding and warding, he has a set that can hide the emerging host under an illusion in case Melkor has a way of scouting from above. He's well-rested. As ready as he's going to be. 

Then they can start marching people through.  

Leareth doesn't need to put anything into the Gate once it's up. He waits. He focuses all of his attention on their surroundings, alert for any sign of an attack, of suspicious magic–

This is not an attack. But it feels like something is - poking the shields around the place where the Gate lets out, on the Endorë side. They're very clumsy. Poke poke.

Hmm. Leareth scans the area for magic. 

Yep! Maia-sized magic, hanging out immaterial right there. Poke poke poke.

Great. Perfect. :Maitimo, there is a Maia on this side that seems to be trying to investigate my Gate: He considers whether it makes sense to respond with immediate hostility. Decides against. :Can you use osanwë with a Maia that is not currently embodied? If not I might try to ask it what it is doing: 

We can use osanwë. What do we want to say to it.

:What is it doing. Nothing more informative of our goals than that, I think. It may be an agent of the enemy, or on the side of the other Valar: less likely since it's not in Valinor, probably, :or neutral. ...Can you tell if Maiar are lying when you speak to them?: Leareth still hasn't attempted properly mindreading one. He has no idea if it would work. Or be safe. 

I wouldn't expect to be able to, not with someone I've never met who doesn't have a physical form. Especially not if they work for the Enemy. - Melkor avoided me so I guess that's some information about whether he thought I'd be able to -

What are you doing?

I'm looking at your magic! I won't break it, don't worry.

Intriguing. Leareth Looks again for a moment, using his scrying-artifact to boost his native mage-sight, trying to memorize the pattern of that nebulous disembodied magic. :Leave it alone, probably. I would prefer they not learn more of our magic, in case they are reporting to Melkor, but I am not sure I can actually do anything to a disembodied Maia aside from the Void trick, and this is not worth revealing that capability for: 

Mhmmm. 

Leareth keeps an eye on it, and watches even more closely for any other magic sources joining it, and waits. Even with a huge Gate and organized people, it's going to take a while to get everyone through. 

No other magic sources join this one. Poke poke poke poke.

Sigh. For the purposes of paranoia, he has to assume the Maia works for Melkor, and is going to learn how his shields work. Fortunately it's the most basic, boring form of magical shielding, and there are other techniques that defend against the obvious routes of attack. He'll remember to switch it up for anything important he builds in the future. 

Otherwise the transfer of the host goes uneventfully.

And Leareth unlinks the Silmarils and tells Fëanáro to take them, and then dismantles the enormous Gate-threshold as fast as possible, puts up the most opaque temporary shielding he can around himself and Maitimo and Fëanáro, does a small regular Gate back to the caves, takes it down the instant they're all through. And he'll spend the next couple of hours scrying around the area to check for any hint that the Gate was traced back to their location.

There are no signs of this. The Maia near the main host drifts off, eventually.

Leareth gets some sleep and then goes to check in with Fëanáro and see if he wants to talk through strategy. 

He's not at a stopping point right when Leareth checks but he is a few hours later. 

"You have a substantial army now," Leareth says. "I was wondering if you wished to talk through plans for how to use it, and if you had questions for me." 

He does have a substantial army! He's very pleased about it. "We want to take back the continent. How would you go about that?"

What a question. 

Leareth gets out paper, though, and starts making suggestions. They'll need to make accurate maps of the area – the locals can help. Figure out which regions are under Melkor's control. Have the army drilling in the meantime; he has suggestions for that, and for how to divide up a chain of command when a number of smaller regiments may end up operating mostly independently in different regions. Suggestions for how to plan around terrain and enemy deployments, and gradually take back chunks of territory until they can pin Melkor mostly to his one corner. At which point they'll need more than just a non-magical army to go any further – but hopefully, by then, they might have something. 

"Yes. This is - mostly all a distraction. But a distraction that gives us more warning if he strikes at us, since he'll have to cut through territory we control, and a distraction that buys a lot of civilian lives, so. - do you think Nelyafinwë is competent to handle it?"