Part of the mind of Ilúvatar

[Inziladun]

I think Ibrin has it on the question of Melkor and Manwë being "coëval".

The Merriam-Webster definition of "coëval is

of the same or equal age, antiquity, or duration

The Valar are all brethren to one another, as evidenced by the quotes Form cited.

I therefore take Melkor's special connection with Manwë to mean that they came into being at the same time; the first creations of Ilúvatar. That would in no way conflict with the statements regarding Melkor being the most powerful of the Valar.