I still maintain that a 4 round cooldown is over the top, given that she is nothing special outside of that ability. You take her for haunt, which people have already worked out plenty of methods to deal with. Another option might be a duration nerf though. This would reduce the damage per haunt cast and it would force the val'kyr player to spend more time switching pets around, losing turns. Mind you I've stopped playing the pet almost completely (only use her when I run a triple undead comp for fun). Fact is that the moment you counter haunt she's not a big threat, that's not something you can really argue with. If you make that ability too risky (deflection has a 3 round CD, I would not take a pet that requires me to win a guessing game with my opponent to even be worth having on my tea) then the pet is simply not worth it.
What she brings right now is a delayed nuke (CoD), good dot with a free swap. She's convenient, easy to play and has low risk. Her strength isn't that she's inherently powerful, it's that she doesn't really have any weaknesses. She has counters, sure, but no inherent weakness. If you give her a huge weakness, then she goes from too good to not worth it. If you're going to gamble there are simply far better pet to do so with (any pet with that a big 50% hit chance move).
As for UA, I think the ghostly skull is a far better option if you intend to go for that ability. The rest of his moveset is a lot better imo.