Hi Bev, I looked at your blog.
I agree, the front looks pretty good. The back is straining and pulling the side seams back. I would say you need a couple of inches extra back width above where the crotch starts to curve in. And a deeper crotch curve. And a flat seat adjustment. And *then*, assess the legs. Oh - also - since crotch depth (waist to crotch length) really can change fit, can you check that the length is right against a pair/pattern that fits well?
Regarding muslins, I really agree (Bev says she's not a muslin devotee). I mean, you can make a muslin to check construction and design details, and I think making the fitting shell pattern is useful, you know, to get an idea about things that are relatively invariant (like shoulder angles, body lengths, etc), but I will NEVER sew up again without basting first. There's just no point, since every fabric is different. In Pants for Real People, Pati Palmer suggests making a note on your fitting pattern of where different fabrics need adjustment to the basic pattern. And boy, do they.