[ "One was very great and deep and beautiful, but it was mingled with an unquenchable sorrow, while the other was now grown to unity and a system of its own, but was loud and vain and arrogant, braying triumphantly against the other as it thought to drown it, yet ever, as it essayed to clash most fearsomely, finding itself but in some manner supplementing or harmonising with its rival". also Compare The Lost Road "The other had grown now to a unity and system, yet an imperfect one, save in so far as derived still from the eldest theme of Il├║vatar...").
2. The Great Music of the Ainur progresses thus: Il├║vatar introduces a First Theme to the choir of the Ainur and Melkor 'spoils' it, converting many of the Maiar to his cause in later years (these fallen Maiar become in the later histories characters like the Balrogs, Ungoliant, and Sauron himself.) Next, Il├║vatar imposes a Second Theme, and again Melkor corrupts it. Il├║vatar then proposes a Third Theme, which is also infected by Melkor and devolves into a raging disharmonious conflict. Il├║vatar puts an end to the disharmony with the conducting of "…one chord, deeper than the Abyss, higher than the Firmament, piercing as the light of the eye of Il├║vatar…" (Ibid.) After the Great Music stops, Il├║vatar promptly chastises Melkor and then leaves the room. This causes Melkor to be ashamed which in its turn causes him to become angry. ]
That's for the first thing.
Basically, Melkor was able to corrupt two separate whole creations of Eru, and infected the third to some degree, creating all the fell concepts that exist within Ea. That's straight from LoTR wiki; although I can go get my Silmarillion later if I absolutely must.
Also, right
here for the answer to the second part, although as I said I can consult an official compendium and the Silmarillion itself if need be.
I objectively think that's enough for at least a Possibly 2-C rating, though this is just my opinion on the matter.