Even if Feanor did agree that the jewels should be broken for the healing of the trees, it is possible that the Noldor would have gone after Melkor for the reason that he had killed their king Finwe.
If the Noldor did not go and follow Morgoth, I think the Valar would have done something in order to save the Sindar and men. One of their reasons for not intervening earlier was that they wanted to punish the noldor for their rebellion and arrogance that they could overcome a valar. Valinor wouldn't have been hidden against any return from middle earth, and so the Teleri, probably under Cirdan could have sent a ship asking for help which they would have given.
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His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
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