
Turin was not even born when the Silmarilli were stolen from Valinor, so no prophecy was there then.In a sense, I believe his physical form would perish for a while and he would lose a bit of his strength overall but given the fact it's prophesied he would come back and be slain by turin. In any case, she was massive and just consumed the light of the two trees, who's power is probably 10000x that of a sun. Now if the balrogs didn't come, morgoth would be in big trouble, what would happen is up to theory.
for some time....... Until ungoliant comes back hungry again. Aren't the light of the two trees and three simarils the same thing (simarils being smaller of course). Such power that allows a spirit or maiar to overcome a Valar, the greatest of which. If she realises that power and becomes ambitious, well there may have been a scarier end to middle earth.Only his physical body would have been harmed.
Ungoliant had the power of harming Melkor unless why would he have been so afraid of her.
But killing Melkor was not possible for her, since Melkor was the most powerful of all the Valar and he could only be killed by doom ordained by Eru alone.
Turin was not even born when the Silmarilli were stolen from Valinor, so no prophecy was there then.
If Ungoliant would have tried to hurt Melkor empowered by the lust for the Silmarils, the events would have been a bit different, maybe there would have reigned peace for sometime in Middle Earth.