That is a lot of hops! Yeah, I generally use just one to three varieties, but that's just me. The only hop I've had some trouble with is Saaz for dry hopping, I just didn't like it and thought it made the beer taste like dirt.
I think I've added up 20oz of hops in your 5 gal batch. That's a lot of hops! I said that twice now. Maybe I stand to be corrected by others with more experience, but my 2xIPA's have at most half that quantity.
BUT...then again, your hop choice might be just fine. Could it be that they're old? I just noticed you said they were "leftovers." Old hops = dirt flavor.
And your last question, I have had beer mellow in the keg. I wouldn't say age, but some time in the cold keg has improved the flavors of some brews.