Aroma Hops v Dry Hops

I am going to make the King Brian Caramel ale as a 11 gallon batch. I want to bottle 5.5 gallons of this with Pumpkin Pie spice, vanilla bean, and molasses. To allow those flavors and aromas to come through I don't want to do the aroma hop edition of Chinook and Willamette.

I'd like to take the other 5.5 and dry hop with the flame out editons of these hops after the initial fermentation is done. ...Will i get the same effect?
...And...can I dry hop in my keg with a Hop Sock?

Yes, you can dry hop in your keg. The closer to serving that you add hops the more aroma you're going to get from them. The compounds responsible for aroma break down during the boil and they are among the first things to go when a beer ages. So if you're concerned about aromas from your adjuncts, I think you're going about it the wrong way.

It's hard to say without knowing the recipe, but based on your description I would still do the aroma hops in the boil and instead cut the dry hop.