First of all thanks for all the suggestions.
1) Am I incorrect in thinking that the second rank of knights will get their lance attacks and then continue to fight (not the second rank cold ones, of course)? I had gotten the impression that the single rank cavalry unit was now obsolete in 8E, or I'd never have considered such a large unit of CoK (my plan back in 7E when I was buying stuff was 2 units of 5, each a bodyguard to my general and sorceress respectively).
The second rank do get their attacks, but the exorbitant cost of such a unit makes them not very viable in 8th, except for flanking to provide a hammer to the anvil of a good spearman unit.
2) I had thought that if the Death Hag was in the unit with the witch elves, she would be the last to die (I may be misunderstanding the casualty rules), and that with I6 if the WE actually get to combat they will unleash a horrific number of poisoned attacks before the enemy strikes. Harder to -get- there, of course, because of the T3, but perhaps my 40k experience is makign me unfairly disregard the 5+ save of other units as basically meaningless, putting the rest of the army in much the same position.
No, the enemy can choose to allocate attacks against her. That's 3 attacks at minimum, with, depending on who you're fighting and whether you're flanking, 3 to 9 more attacks coming her way. 2W with T3 is easy to rend apart. The Witch Elves in a large unit will have to have the support of a Cauldron with 5+ ward save to ever survive well enough. Better to have them in small units or 10ish models for flanking attacks.
3) hadn't realized you couldn't have 2 arcane items. Good catch.
4) I originally took the Sword of Ruin anticipating challenges from equivalent enemies (the sort that go looking for a unit of CoK) - would a hydra blade be better?
In the new edition, there's plenty of stuff to snipe out characters with expensive items. I would suggest looking a bit lower down in points value. Whip of Agony now nets you an extra attack after the FAQ, and Crimson Death has always been a favorite. Both are quite cheap and provide some punch.
5) I only have 13 blackguard (command set, regular set and a blister with 3) - I get the impression that 2 ranks isn't really enough? Do I need to bite the expensive bullet and bulk up to 20?
Since Black-Guard are Stubborn and built to fight to the last man, you actually do have enough to run them. I would still bulk up to 20. They're still the best Special choice Dark Elves have.
6) I had originally planned on 2 units of 5 harpies for roles like pulling out Goblin Fanatics, targetting war machines or solitary wizards, flying behind units etc.. Is 2 units of 5 good, or just 1 unit of 6 and keep the points for something else?
Their value has diminished somewhat, as they can't marchblock or redirect nearly as well as in 7th. However, they are still good as Warmachine hunters, and they now have the added role of hunting down remnants of units trying to deny you VP's. I think that lowering the amount is debatable, but a 6-man unit is the best for hunting warmachines, getting 9 attacks against what should be 2 or 3 models with light armor will net you a nice good amount of VP's.
I had never really considered hydra's necessary (interesting, but not necessary). It appears I need a rethink. Will return with amended list...