Hi Anthony,
No, I don't think they would have found it was in the Music from the beginning. The Valar seemed to know almost nothing about the Music where it pertained to the Children of Illuvitar. That was opaque to them. So, Manwe would have had to make his own decision on this.
From the Ainulindalë:
"And many other things Ilúvatar spoke to the Ainur at that time, and because of their memory of his words, and the knowledge that each has of the music that he himself made, the Ainur know much of what was, and is, and is to come, and few things are unseen by them. Yet some things there are that they cannot see, neither alone nor taking counsel together; for to none but himself has Ilúvatar revealed all that he has in store, and in every age there come forth things that are new and have no foretelling, for they do not proceed from the past. And so it was that as this vision of the World was played before them, the Ainur saw that it contained things which they had not thought. And they saw with amazement the coming of the Children of Ilúvatar, and the habitation that was prepared for them; and they perceived that they themselves in the labour of their music had been busy with the preparation of this dwelling, and yet knew not that it had any purpose beyond its own beauty. For the Children of Ilúvatar were conceived by him alone;
and they came with the third theme, and were not in the theme which Ilúvatar propounded at the beginning, and none of the Ainur had part in their making.
Therefore when they beheld them, the more did they love them, being things other than themselves, strange and free, wherein they saw the mind of Ilúvatar reflected anew, and learned yet a little more of his wisdom, which otherwise had been hidden even from the Ainur." emphasis added
And from the Quenta Silmarillion:
"Now all is said concerning the manner of the Earth and its rulers in the beginning of days, and ere the world became such as the Children of Ilúvatar have known it.
For Elves and Men are the Children of Ilúvatar; and since they understood not fully that theme by which the Children entered into the Music, none of the Ainur dared to add anything to their fashion. For which reason the Valar are to these kindreds rather their elders and their chieftains than their masters; and if ever in their dealings with Elves and Men the Ainur have endeavoured to force them when they would not be guided, seldom has this turned to good, howsoever good the intent. The dealings of the Ainur have indeed been mostly with the Elves, for Ilúvatar made them more like in nature to the Ainur, though less in might and stature; whereas to Men he gave strange gifts.
For it is said that after the departure of the Valar there was silence, and for an age Ilúvatar sat alone in thought. Then he spoke and said: 'Behold I love the Earth, which shall be a mansion for the Quendi and the Atani! But the Quendi shall be the fairest of all earthly creatures, and they shall have and shall conceive and bring forth more beauty than all my Children; and they shall have the greater bliss in this world. But to the Atani I will give a new gift.' Therefore he willed that the hearts of Men should seek beyond the world and should find no rest therein; but they should have a virtue to shape their life, amid the powers and chances of the world, beyond the Music of the Ainur, which is as fate to all things else; and of their operation everything should be, in form and deed, completed, and the world fulfilled unto the last and smallest." emphasis added
That doesn't sound like opacity, merely lacking detail and only at the far end of the work.
It seems to me that this indicates that there are details of the Music that they don't recognise the import of, until the time of it is right in front of them. This might serve to prevent the Valar from interfering in the most critical pieces of the work, e.g. sending Eagles to Bag-End to fly Frodo to Mordor ;-)
It also indicates that such a decision would not be taken unilaterally by the Valar, but instead in reference to the Music.
All the same, all of this is speculation as there is insufficient textual basis to make a
firm statement.