I assume only High Elves and the Empire call him "The Witch King"? Very few people are on a first-name basis with him so "Malekith" doesn't seem fitting.

What is the appropriate (as far as sMalekith is concerned) long form way to address Malketih? The True Heir of Anerion? The Rightful Ruler of All Elves? What is the short form? "Your Majest"y or is there something more Druchii sounding?

How is Malekith referred to in conversation by Dark Elves when Malekith (or people like the Black Guard) is not around? How do Dark Elves who dislike Malekith refer to him privately?

Repeat the questions above for Morathi, Hellebron, and the lords of the major cities.

Mr. Malekith
Lady Morathi
My girl Hellebron
My boy Malus
Dr. Tullaris
Sir Kouran
Shady Shadowblade
The Corsair formerly known as Lokhir

In ET: Malekith, Kouran always addresses him as "my king," and calls him by his name only once, for the first time ever, for plot significant dialogue which I will not spoil here. I would assume "my king," "my lord," etc would all be common phrases.

I wonder what tortures Malekith would put someone through who said "yo my bro mali!" at court...

I read Khaine so there is no spoilers there, but the only people who really talk to him besides Ulthuani elves are Kouran (who is the only member of Malekith's 'I don't distrust you' list) and Morathi who gave birth to him. I imagine both Kouran and Morathi, being more intimate with Malekith than anyone else, would be able to use simpler forms of address than the hoi poi of Dark Elfdom.

Now that is a name you can trust

[+] SPOILER