Further information regarding the three points below.

We use ONE! aoe stun from either a reti paladin or warlock. That stun gets casted as soon as all 3 ads is target-able. just to freeze them in place for a brief moment.

together with the stun going of all dps picks a Valkyrie and dps it down, switching to always be dpsing the Valkyrie that is closest to the edge, no assigned groups anymore.

We'll have a few single target stunner's assigned to different ads, such as rogues and paladins. to not cause Dim returns to quickly

The main thing here we need to get the raid to learn is to NOT use stuns when you are not supposed to, excessive use of stuns means we trigger diminishing returns and that's bad, in short it will make the ads stunned less time then if we use stuns in the right way.
So only ONE aoe stunned used per Valkyrie spawn, this is due to the stun only being 3sec, meaning with dim returns it will only last 1.5sec whilst there are other single target stuns that lasts for 6sec (3sec dim returns) +1.5sec extra stun time on each ad will make a BIG difference.

In matters of slow we will use slowing traps from now on as well, reason being that slowing trap is the only slowing mechanism that we can be 100% sure that the slow is always going to be on the ads, reason being that higher slowing ability's remove less slowing ability's and with that we can end up with an ad going full speed to the edge.

Example, We have a fury warrior with Pricing howl that slows ads for 50%, he slows all the ads then a DK puts a chains of ice on the target, as chains of ice slows more then 50% the pricing howl slow will be removed from the ad and adding to that when the warrior try to use pricing howl on the ad again it will not be applied as 'an more powerful effect is already active'. When the chains of ice then times of the target there will be no other slow on the ad and it will race full speed to the edge. With a trap down that will never happen.

Since we are positioning the direction of where the Valkyries are traveling away from any current defile we should not have any problems with a trap overlapping the graphic of a defile.