Your best bet is to use them for flavor and aroma, since then you can use them as is. If you wanted to use them for bittering you'd have to find some way of measuring the bittering acids, such as boiling in a light sugar solution for 30 mins, doing the same with another known AA variety and comparing/diluting until you can get some idea of the bitterness.
Its possible that the added vegetal material will be an issue in a style as delicate as Kölsch. But like you mention, I too have loaded up beers with more than twice your proposed amount and been OK (albeit not in a Kölsch).
What would worry me more is that 2.3% Alpha Acids is really on the low end for Hallertau. I'd be worried about the quality of the ...
Dry-hopping does not impart bitterness.
The bitterness in the beer originates in the isomerization of hop alpha acids in the heat of the boil.
Since dry hopping is done cold, no AAU units of bitterness are generated.
There are a bunch of factors to consider here. To name a few:
As you mention, zero to very little gravity will tend to increase the utilization rate as there will be less competitive inhibition from wort sugars.
Boiling in water alone will mean a higher pH (as malt phosphates, even in extract brewing, would normally react with hardness in/added to the water ...
The AA content of hops is seasonal, and depends a lot on the weather. What you see listed in books is "typical", but that doesn't mean it can't be higher or lower than that. You should always adjust any recipe to the AA of the hops you;re using. Don't match weights of hops, match IBU contributions.
Since you are already substituting cascade hops instead of centennial hops, you are not going to hit some pre-destined goal. So you might as well go with personal preference. If, when you drink an APA or IPA, you don't like a lingering bitter, then cut back proportionally on the bittering addition (usually the 60 minute addition). If you like the hop ...
You get so little bittering from hops added at 15 min. or less that I keep those amounts constant. Obviously, the IBU won't be identical, but it's close enough that you won't really notice the difference.
I use the IBU calculator at Brewer's Friend, it's quite quick and nifty. It also helps me as I have a different boil gravity than fermenter gravity and this effects the impact that the hops has.
It does however not deal with anything but alpha acids, the aroma I would believe is rather more difficult/unfeasible to calculate to satisfaction.