The
Valar were not inactive during the Second and Third
Ages. They sent a considerable embassy to Tar-Atanamir in hopes of stemming what blossomed into the
Númenórean Apostasy against
Eru and eventually into
Ar-Pharazôn’s attempt to invade and conquer
Valinor. In
the Third Age, they at least sent the five
Istari (and possibly others) to counter the reärising of
Sauron, of which they were aware by the end of the first millennium.
Elbereth heard the prayers of
Elves and
Men in
Middle-earth, and whether by chance or fate or intervention, these seem sometimes to have been answered. They were not merely resting on their tushies, though I suppose they did a lot of that, too.
As for Eru appearing in
Arda,
Finrod and Andreth discussed this:
[Andreth said,] ”How or when shall healing come? … [W]hat of us who before it go out into darkness unhealed? To such questions only those of the ‘Old Hope’ … have any guess of an answer.”
… Finrod [said], “Who are they?”
… “They say,” answered Andreth, “that
the One will himself enter into Arda, and heal Men and all the Marring from the beginning to the end. This they say ... is a rumor that has come down through years uncounted, even from the days of our undoing. … Men perceive the world only as a war between Light and Dark equipotent. But you will say: nay, that is
Manwë and
Melkor; Eru is above them. Is then Eru only the greatest of the Valar, a great
god among gods, as most Men will say, even among the
Atani: a king who dwells far from his kingdom and leaves lesser princes to do here much as they will? Again you say: nay, Eru is One, alone without peer, and He made Eä, and is beyond it; and the Valar are greater than we, but yet no nearer to His majesty. Is this not so?”
“Yes,” said Finrod. “We say this, and the Valar … say the same…”
… Andreth [said], “… How could Eru enter into the thing that He has made, and that which He is beyond measure greater? Can the singer enter into his tale or the designer into his picture?“
“He is already in it, as well as outside,” said Finrod. “But indeed the ‘in-dwelling’ and the ‘out-living’ are not … the same … .”
“Truly,” said Andreth. “So may Eru … be present in Eä that proceeded from Him. But they speak of Eru Himself entering into Arda, and that is a thing wholly different. How could He the greater do this? Would it not shatter Arda, or indeed all Eä?”
… Finrod [said], “These things are beyond the ... wisdom of the Eldar, or of the Valar maybe. … But such words may not be used of the Measureless. If Eru wished to do this, I do not doubt that He would find a way, though I cannot foresee it. For, as it seems to me, even if He in Himself were to enter in, He must still remain also as He is: the Author without. And yet, Andreth, to speak with humility, I cannot conceive how else this healing could be achieved. Since Eru will surely not suffer Melkor to turn the world to his own will and to triumph in the end. Yet there is no power conceivable greater than Melkor save Eru only. Therefore Eru, if He will not relinquish His work to Melkor, who must else proceed to mastery, then Eru must come in to conquer him.”
This I cite from “Athrabeth Finrod ah Andreth” (“The Debate of Finrod and Andreth”) in
Morgoth’s Ring. The section is worth reading, if not for this exchange, then for the sad but beautiful star-crossed love story of Andreth, a mortal woman (and the aunt of Barahir,
Beren One-handed’s father), and Finrod’s brother,
Aegnor.