I found this thread while looking for a fix to the mouse controls of Powerslave Ex. Sadly I found none. The good news is that Nightdive which has hired this guy and brought out games like Turok Ex on PC will release Powerslave Ex (and Forsaken Ex and others) officially as was recently revealed so I assume that version will be improved and fixed. You can always play the existing Powerslave Ex with a gamepad, that doesn't present the mouse control issues and is more faithful to the original.
However, you sir are a liar! The PC "version" (if you can call it that, it's a wholly different experience using the same sprites/textures/music/etc.) is indeed quite awful and inferior to the PlayStation version, however the Saturn version is arguably superior as it includes more elaborate level design that allows for sequence breaking thanks to the ability to "grenade jump" with a certain weapon, something you can't do in the PlayStation version. It only lacks some detail geometry in places but more than makes up for it with often larger spaces and sections the PlayStation version is missing altogether. Plus the basic crab enemy is a neater red sprite instead of the PlayStation's blue scorpions
Way better video for the game and its various versions courtesy of Digital Foundry Retro:https://www.youtube.com/watch?v=HCsO8O4W4Fw
I'm hoping the official Powerslave Ex release will include both the PlayStation and Saturn campaigns so you can appreciate the superioty of the latter. Alternatively it should only include the Saturn version but that's not likely seeing as the previous unofficial release only had the PlayStation version. Either way the real game (so, either PSX or Saturn, not PC) is a must play for every old school FPS fan, although you can't really call it old school given its design was so far ahead of its time and only revisited in far more recent games like the Metroid Prime series.