I, too, read "coeval" as "equal in power" when I first read Silmarillion (also as a teenager when it first came out). It was only later I learned more about "coeval" and I think now that Tolkien really did understand it's nuance and use it intentionally meaning that they were "created" together as (dare I suggest) non-identical twins - one of whom had greater gifts but was motivated by increasing his own position and power, while the other was wiser and more humble (more willing to seek the glory of his father, rather than his own).
Actually, I wonder now if Tolkien's knowledge of the twins Esau & Jacob may have informed his subcreation of Melkor & Manwe. [note, I don't say inspired - but may have provided insights he used in giving them their qualities]
A few related quotes:
- "Mightiest among (the Ainur) is Melkor" (implying mightier than Manwe - though not, necessarily, more noble in spirit).
- "In the powers and knowledge of all the other Valar (Melkor) had part". Not said of Manwe - implying Melkor has more raw strength and knowledge than Manwe.
- "Manwe and Melkor" were bretheren in the thought of Illuvatar" even though "mightiest of those who came into the World was, in his beginning, Melkor."
- In the Valaquenta it's interesting that Tulkas is the strongest of the Valar (implying stronger than Manwe) even though Melkor is called the Mightiest of the Ainur (therefore also of the Valar who were a subset of Ainur) - hence stronger than Tulkas (at least, in his beginnings). Implying again that Melkor was stronger than Manwe.
Also, I seem to recall something in one of the HoME books about Manwe, after Melkor's first defeat at Utumno, being surprised how easily Melkor was beaten - not realizing how his power had become disbursed in the mastering of Arda and expecting him to be FAR more powerful then himself.