How do I lessen the effects of thickening caused by roux?
I believe that rather than "diluting" your gravy with stock, you could instead use less roux (fat and flour) with the same amount of juices (and perhaps a bit of stock).

The extra tablespoon of butter, for example, meant you were "bumping up" the thickness of your final gravy to the next level. Here's some typical ratios from an earlier Question I had about what makes a white sauce "medium":


  Medium refers to the thickness level of the final sauce, and is
  controlled by the ratio of roux (butter/flour) to [liquid]. For 1 cup of
  [liquid]:
  
  thin = 1 tablespoon each flour/butter
  medium = 2 tablespoons each flour/butter
  thick = 3 tablespoons each flour/butter


So instead of using 2.5 tablespoons of fat and 2.5 tablespoons of flour, you could leave out the butter; a 1.5/1.5 tablespoon roux with the same volume of liquid might give you a texture closer to what you're hoping for.