Some updates (I find it helpful to post my experiences for feedback):
I've decided to abandon half-lotus position in favor of full-lotus or Burmese. Half-lotus seems very unstable to me (my unsupported leg often starts to shake), since there are only two points of contact with my sitting surface (one knee and the pelvis). I think moving to a three-contact position will help straighten my spine some, without requiring me to exert effort to maintain the spine. I am still having some issues with the numbness. Today, I was perusing the Internet Buddhist community for answers, and I found one on ZFI. Apparently, the numbness is caused by pressure being placed on the aciatic nerve, which is located in the hip along the curve of the butt. On ZFI, a user on the post I read mentioned that this issue can be helped by stretching before meditation, elevating the butt a few inches (all ready do this), and by positioning the legs and then moving the seat under the butt as opposed to sitting and then positioning the legs. In the mean time, I will try to maintain focus on the breath.
I also read today that anapanasati should not become too focused on the nose because such a fixation can cause us to ignore the part of anapanasati which calls for "breathing sensitive to the entire body." I think I can confirm this from personal experience. I have tried a couple of different points of focus. I find that whenever I center my focus on the space in between my eyes (I think of this as literally setting my mindfulness "to the fore"), my concentration often improves and I find it easier to relax my body.
Any advice concerning posture? Meditation practice is truly turning out to be an experiment, as everyone says.
I do not want my house to be walled in on sides and my windows to be stuffed. I want the cultures of all the lands to be blown about my house as freely as possible. But I refuse to be blown off my feet by any.- Gandhi
With persistence aroused for the highest goal's attainment, with mind unsmeared, not lazy in action, firm in effort, with steadfastness & strength arisen, wander alone like a rhinoceros.
Not neglecting seclusion, absorption, constantly living the Dhamma in line with the Dhamma, comprehending the danger in states of becoming, wander alone like a rhinoceros.- Snp. 1.3