Quick question pertaining to the usage of Bomret's Detailed Normals and this mod...
That should work fine. If you have both packages in OMOD form, and you activate Bomret's first, then Detailed Terrain, you should be prompted to overwrite the landscape normal maps. Doing so should produce grand results.

Would installing qarls 2 and 3 make this look any better?
As Bomret said, the landscape textures used by this mod are already based on the QTP3 textures. Using either of those texture replacers would improve the buildings and dungeons and such, but wouldn't effect the landscape since DT already brings it up to QTP3 standard (and beyond).
...in which sewers and castle walls were off kilter due to this particular mismatch... Any ideas?
I'm not sure what could be happening there, but it sure sounds like you're just missing some of the textures from your QTP installation. That could be a problem with the file you downloaded, the unpacking process, lots of things really. You'll probably have better luck asking in the QTP thread.