I agree with your point about Nirnaeth being inevitable. But your point about the Valar being at fault...well...
Quote:
...and a new theme began amid the storm, like and yet unlike to the former them, and it gathered power and had new beauty. But the discord of Melkor rose in uproar and contended with it, and again there was a war of sound [there being a battle of music between the Ainur and Melkor's theme before] more violent than before, until many of the Ainur were dismayed and sang no longer, and Melkor had the mastery.
(The Silmarillion, Ainulindalė - emphasis mine)
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The Ainur were also actualising their role in the conflict against Melkor - most of them sat it out, except, as examples, Melian and Ulmo. For a time, as the Eldar gained power and beauty in Arda, Morgoth's hatred will be poured out on them, and, for a while, they will be overpowered. The Valar not participating in that fight was pre-ordained by the Music, so any blame could be excused as them only doing what they could do, bound to the Music as they are.