some parts of the code are shared between the three, but my guess would be SS2/Dark 1.5 only.

When TFix detects whether or not you are using a copy of Thief Gold/Dark Project, does it discern between the different map files as well? I'm trying to compare my TG install with TDP but can't spot the texture differences between the two Bonehoards and using Youtube doesn't help as most videos are mislabeled.

EDIT: After playing more, there was a fire shadow in Bonehoard. Was able to fix it though by finding an article (can't post links on GOG forums despite having a decade old account) to get the installer working instead of trying something hacky like pasting the thief folder from the CD then dumping tfix lite on top.

yes, TFixed TDP install is basically T2 running TG, which is pretending to be TDP. go for TFix lite if you want the original TDP levels completely unchanged.