So on the default keybinding, you've got a semi-WASD setup, except that S isn't walk backwards. That's X. S instead is a very slow creep forward. However, this creep forward does not appear in the keybinds anywhere at all. So if I want to rebind S to be walk backwards, I lose that creep forward. It's not the same thing as holding shift and walking forward, either. It's a completely different kind of forward creep. Slower and quieter. Is there any way to rebind that slow creep to something else? I've had a look in the DARK.BND to see if it was a hidden keybind, but it's not there, either.

Based on an untouched fresh install (from GoG), here are settings in DARK.BND
Clearly shows what you have to set I think. Anything with "fast" seems to be the normal movement. the just "forward" seems to be the one you want to set to something else.

bind shift +creepon
bind alt +slideon

bind w +walkfast
bind keypad_up +walkfast
bind up +walkfast

bind s +forward
bind keypad_center +forward

bind x +backfast
bind keypad_down +backfast
bind down +backfast

bind z +moveleftfast
bind keypad_end +moveleftfast

bind c +moverightfast
bind keypad_pgdn +moverightfast

bind a +turnleftfast
bind keypad_left +turnleftfast
bind left +turnleftfast

bind d +turnrightfast
bind keypad_right +turnrightfast
bind right +turnrightfast

Lol, also found this in the DARK.BND. Pre-setup cheat?

bind end+alt+ctrl+shift win_mission