If she has CCN she would be dead by now, I think a cyst in her brain - as you have observed tapeworm in the past - is a likely scenario. Vet wasn't overly concerned when I found tapeworm in one of my goats, but treated her (and everyone else) with white wormer (Panacur is the only one to kill tapeworm) and never had the problem again.
Liseriosis also attacks the brain, but the open eye would be unusual, usually the eye lid/ear droops on the affected side, and she would drool a lot. But a 4 year old is a bit older than what you would normally expect - it is usually younger animals with gappy teeth (open places where bacteria can enter) that are more likely to be affected.
Not sure this will have a good outcome...whatever it is. But probably an individual sheep problem not a flock one, and in either case not much you could have done about it.