Interesting that your months old frozen henna seems to give you the same color ... that's good to know
Yeah I agree about the need for more reliable info on henna. As far as darkening your color, my understanding is you can achieve that one of two ways (according to hennaforhair.com):
1. Doing multiple henna treatments over the same hair. Drawback: possible curl loosening or loss of curl. I suppose that's why you only want to do your roots at this point?
2. Using a henna with a high percentage of lawsone. The higher the lawsone content, the deeper the red, and the less orange/brassy the outcome.
If, for example, you wish to color your roots something in the neighborhood of a rich auburn/burgundy (for medium to dark brown roots), you could use henna w/a lawsone content of about 2.6 or higher and get that richer, darker color from just one application. Red Raj, for example (via Henna Sooq) has 3.2 and Raj Twilight has 2.6 (from mehandi.com / Hennaforhair). I actually am aiming for a lighter color on my roots so I can choose something like Jamila henna or Punjabi (lawsone lower than 2.0) for the future, though gray coverage might be iffy. For now, I'm just diluting my Twilight Raj w/conditioner instead, which will save me money and work effectively (as reflected in strand test outcome). ... Each time I do my roots, I'm adding 25% amla to my mixture to help maintain the tightness of my curls. Amla may be worth trying if you don't want your curls loosened.