Give the full dose. This is therapeutic not simply nutritional. Excess will be expelled in the urates. There's no immediate danger of overdose. Keep it up until you notice improvement, then continue for a few days beyond that.
Yes, continue the E. You can get some selenium tablets and give a sliver when you give the E capsule. You can give the E twice and day or even three times a day. Include B-complex as that can help repair any damaged nerve connections.
Nutri-drench is great. Go ahead and give it, either mixed in water so it appears as "weak tea" or give it right from the dropper undiluted as many drops as the duckling will drink off the end of the dropper.
But it's not enough to cure wry neck. Add one capsule of vitamin E400iu each day...