[Maédhros]

I do wonder about the extent to which the Valar's inaction is due to them trying to 'actualise the Music' - which they are bound to do, IMO, & how much is down to shock, confusion & self doubt. They thought they were doing the right thing up to then, & suddenly it all collapses like a pack of cards.

I have to say that I totally disagree with this notion.
From Vinyar Tengwar: Ósanwe Kenta

If we speak last of the "folly" of Manwë and the weakness and unwariness of the Valar, let us beware how we judge. In the histories, indeed, we may be amazed and grieved to read how (seemingly) Melkor deceived and cozened others, and how even Manwë appears at times almost a simpleton compared with him: as if a kind but unwise father were treating a wayward child who would assuredly in time perceive the error of his ways. Whereas we, looking on and knowing the outcome, see now that Melkor knew well the error of his ways, but was fixed in them by hate and pride beyond return. He could read the mind of Manwë, for the door was open; but his own mind was false and even if the door seemed open, there were doors of iron within closed for ever.
How otherwise would you have it? Should Manwë and the Valar meet secrecy with subterfuge, treachery with falsehood, lies with more lies? If Melkor would usurp their rights, should they deny his? Can hate overcome hate? Nay, Manwë was wiser; or being ever open to Eru he did His will, which is more than wisdom. He was ever open because he had nothing to conceal, no thought that it was harmful for any to know, if they could comprehend it. Indeed Melkor knew his will without questioning it; and he knew that Manwe was bound by the commands and injunctions of Eru, and would do this or abstain from that in accordance with them, always, even knowing that Melkor would break them as it suited his purpose. Thus the merciless will ever count on mercy, and the liars make use of truth; for if mercy and truth are withheld from the cruel and the lying, they have ceased to be honoured.

At that point they suddenly cease to do anything effective against Melkor, as if they're afraid that any major action will blow up in their faces, so they wait until they feel they know what they should do - but lets face it, if it hadn't been for the Noldor in ME Morgoth would have had a free rein to do whatever he wanted. The Valar, in their confusion, & desire to see the Noldor humbled, desert Men, Dwarves & Sindarin Elves.

While it is true that the Valar seemed to desert Men, it appears that it was not so.
From Morgoth's Ring: Athrabeth Finrod Ah Andreth. The Tale of Adanel

Some say the Disaster happened at the beginning of the history of our people, before any had yet died. The Voice had spoken to us, and we had listened. The Voice said: 'Ye are my children. I have sent you to dwell here. In time ye will inherit all this Earth, but first ye must be children and learn. Call on me and I shall hear; for I am watching over you.'
We understood the Voice in our hearts, though we had no words yet. Then the desire for words awoke in us, and we began to make them. But we were few, and the world was wide and strange. Though we greatly desired to understand, learning was difficult, and the making of words was slow.
In that time we called often and the Voice answered. But it seldom answered our questions, saying only: 'First seek to find the answer for yourselves. For ye will have joy in the finding, and so grow from childhood and become wise. Do not seek to leave childhood before your time.'
But we were in haste, and we desired to order things to our will; and the shapes of many things that we wished to make awoke in our minds. Therefore we spoke less and less to the Voice.
Then one appeared among us, in our own form visible, but greater and more beautiful; and he said that he had come out of pity. 'Ye should not have been left alone and uninstructed,' he said. 'The world is full of marvellous riches which knowledge can unlock. Ye could have food more abundant and more delicious than the poor things that ye now eat. Ye could have dwellings of ease, in which ye could keep light and shut out the night. Ye could be clad even as I.' ...
The first Voice we never heard again, save once. In the stillness of the night It spoke, saying: 'Ye have abjured Me, but ye remain Mine. I gave you life. Now it shall be shortened, and each of you in a little while shall come to Me, to learn who is your Lord: the one ye worship, or I who made him.'

It seems that the Valar could not come to the aid of Men as when the Elves came because Men were counselled by Eru himself. Now of course, when one reads the Published Silmarillion it is understandable to come up with the idea that after the Fall of the Ñoldor, the Valar left all of ME for the Ñoldor to battle Morgoth. It is indeed IMO, that the Sindar elves and the dwarves were unjustly punished in a matter between the Ñoldor and the Valar though.

One has to question the idea of what would have happened if the Valar had fought earlier with Morgoth? Look that most of Beleriand had been destroyed in the War of Wrath. Can you imagine how much more of ME would have been destroyed in an earlier battle? Morgoth as a being, would have had more inherent power in itself than he had at a later point. IMO, much more of Arda would have been destroyed.

Or course, the Valar themselves are not perfect an even they lacked estel.
From Morgoth's Ring: Converse of Manwë and Eru

but it was not until the death of Míriel in Aman that Manwë appealed directly to Eru for counsel. Eru 'accepted and ratified the position' - though making it plain to Manwë that the Valar should have contested Melkor's domination of Middle-earth far earlier, and that they had lacked estel: they should have trusted that in a legitimate war Eru would not have permitted Melkor so greatly to damage Arda that the Children could not come, or could not inhabit it.