K - Before the blackness, seek their doom.
H - The slender bough bears a unique gift, but what meaning does it hold?
U - An elder claimed this eldest title, and sought the eldest's crown, but the eldest's servant ended him.
Zirak-Zigil - Here ended the endless; what did the makers make it?
Durin - Seven times named, but twice unnamed; what name is mine but never was?
U - A son full-grown bears his father's name in full.
L - Fall back, o masked one, and let the French striplings take the field.
That was impressively quick! I guess there aren't all that many words with a Z in the middle...
The theme is actually a bit more specific than you suggest; I'll let you keep working on that.
For
Durin - there are seven Durins, but we don't know either their Khuzdul names, nor their 'actual' Middle-earth names - 'Durin' is Old Norse, not Old Westron. So they were 'unnamed' twice, and the name they actually have was never 'really' used.
hS