Wow, that looks fantastic!
I do remember the seamless cubemap texture problem, but only from the GL days. There was an extension to deal with it in GL...
https://developer.download.nvidia.com/opengl/specs/GL_ARB_seamless_cubemap_per_t...
...but according to a quick google, D3D didn't get a fix for this until D3D9-ish.
I can't think of a way you could fudge this by distorting cube faces either so sorry but I think you're stuck with it in B3D.
But it all looks pretty seamless from here anyway!