Let's Talk Morgoth

[Kuruharan]

but he also wasn't damned in any way as he would have been if he had managed to sabotage Iluvatar's purpose.

I think sabotaging Iluvatar's purpose was beyond Melkor's capacity.

Also:

But Manwe was the brother of Melkor in the mind of Iluvatar, and was the chief instrument of the second theme that Iluvatar had raised up against the discord of Melkor

Iluvatar evidently felt the discord was something to oppose.

I personally don't understand how Iluvatar's actions could be interpreted as favorable toward Melkor's activities. Note that Melkor was filled with shame as if he had been rebuked (which I think he had been).