Ok, so I got home and opened up my scene and it was fubar again : /
After digging around, I think I’ve identified the issue, and it appears to be (at least in part) a blender “feature”:
If you google this problem, a number of posts tell you to set the normals image texture node to Non-Color Data.
By pure luck, I noticed that Color Space over in the properties panel (for the material node window) was still saying sRGB.
After setting this to non-color as well, my normal map now displays properly.
<bangs head on table several times>
Makes sense now, but very obscure thing to find (like I said, pure luck).
Might want to look into that blender developers…