Wasn't aware of the 0.4.9 release, much appreciated! That solved the texture issue.
Tried running dernc manually on the old KeeperFX and Gold install, and it successfully decompressed the texture files that previously failed. I've ran Unearth outside Program Files and a folder structure without spaces or special characters, but that didn't help. It's always the same files that fails to decompress.
The only reason I asked about Python was because a text editor was set as default for .py files, and it opened when running decompression through Unearth the first time. My setup with Python is quite unorthodox, so figured it might be the issue.