After reading all the advice here and sleeping on it, I've decided to keep the jeans. Thank you all so much!
I don't currently own any mid-calf or moto boots, but it sounds like a good purchase to plan for.
Diana, I very much agree: a little scrunch goes a long way to help even things out.
Cocolion, thank you so much for those links! I don't see any I could reasonably fit into right now, but it's good information to have for the future. (I'm currently sitting between size 14 and 16, with a 34-35 inch waist.)
Karie, I have indeed tried the jeans at Gap and Old Navy and ran into the same issue with both: way too loose in the waist and hips (to the point that no fabric is in contact with my body there, and the area is so loose that the fabric can't hold itself up) and too tight in the thighs, usually with not a long enough rise for me. I've been too large for JCrew so far, although I think I'm starting to approach their size 14 territory (for whatever reason, they don't offer their tall lengths in 16). I haven't tried BR jeans yet, but I should, since I've liked trousers and pants I've bought from them in tall lengths.
NYDJ often offers me a good fit in the waist/hip/thigh and their tall lengths are usually long enough; the problem in this case was that I didn't like their tall skinnies, and the pair I did like they don't make in a tall length. I've had some luck with Coldwater Creek's long lengths, but their fabrics often don't feel sturdy enough to me. I've also tried CJ by Cookie Johnson with success. (Including this for other women who may run into the same issues I do.)
I'm still too big for most premium denim brands, but getting ever closer to the point that I'll fit into their size ranges and suddenly have much more choice.
Man, I should make a spreadsheet of brands I've tried and the fit issues I've had with each and in what sizes. (I probably still have all my fit notes from the last time I did a massive denim purchase for this exact purpose.)