An interesting combination of prompts and a neat way to respond to them!

You gave these two a really distinctive style of communication here, especially Sauron's use of relationship terms to refer to  Ainur.

It seems to be alluding to the earlier concepts of the Maiar as children of the Valar, but here it sounds a bit more like honorifics, perhaps.

....how cringingly apologetic Sauron is, including the attempt to elevate Lúthien and Huan's power, power that Melkor doesn't acknowledge. They and Beren managed to take one Silmaril away, and could have had more (if the curse of the Noldor hadn't struck Curufin's knife). Perhaps that deflection is why Sauron is so quickly forgiven. 

I really love that the Valar have parental epithets, and the Maiar are referred to as siblings. 

Oh, this very awkward, not to mention embarrassing, for both of them. The different ways they address each other is also very telling. 

I like that Sauron thinks of Melian as his sister, and it's funny how much Melkor doesn't understand what that means about Luthien's power. 

I like Mairon’s verbose explanation just after Melkor has asked him to do it swiftly. Melkor’s description of the situation is much shorter and lacks all the excuses Mairon weaves into his.

I wouldn't acknowledge that I fell asleep to the same girl's lullaby either, if I were Morgoth! ;)