I've been making a special point to spend quite a bit of time playing in the low register. However I haven't noticed much change down there after about two months of practicing pretty much every day. The tone color is getting a little better in the medium-low register (first-line E down to G or F), but the lower third partials and second partials are still hard to sound strongly without a bit of embouchure tweaking, and the timbre and intonation are very difficult to control when I do this.

Am I doing something wrong in my practice sessions (above and beyond cheating on the embouchure) that is hindering progress down there? Or is it normal for volume, tone quality and articulation to improve very, very slowly down there? I'm also interested in some good low note training exercises.

I wouldn't answer this question directly with a 10 foot pole.

Generally, getting volume ou tof the low low notes takes sometime and proper exercises. Getting more notes to your range once you get down to the A (bottom space bass clef) progress any lower can take forever - - especially without a competant qualified teacher.
There are a bunch of professional players who don't play below the A; so don't think that it should be easy.

Believe it or not, during today's practice session I noticed that the 'alto clef' notes (midrange G down to low F) had a nicer tone quality than I think they did a couple weeks ago, and while the notes below F still are rather tricky, they seem to be getting at least somewhat stronger and easier now. I've been doing some exercises for low register articulation as well as exercises that take you up and down through a wide swath of the horn's range.

I recently heard an audio recording of a natural horn playing its harmonic series, and I remember how beautiful the low notes (second and third partials) sounded. Perhaps I'm setting the bar too high with respect to tone quality, at least...