lewuz: Late to help but the mouse stuttering should be fixed if you set your USB polling rate to 125Hz. Modern mice polling rates are up to 1000Hz which was unheard back in the day. Most gaming mice software allow to downgrade it.
for some reason this didn't do anything for me, although it sounded like it made sense. well, maybe it did do something (couldn't tell), but certainly not enough to make mouse movement feel good enough.
how i ultimately "fixed" the mouse movement, was setting both "maxfps" and "inputrate" to 60 in the autoexec.cfg, and now my mouse movement feels alright.
i keep seeing the number 50 pop up, rather than 60.. makes me wonder if the engine was optimized for 50fps specifically?
though, most settings default to 30 in this game, which could mean that it was actually optimized for 30.
anyway - "inputrate" in the autoexec file refers to the in-game "smoothness" option. if you use the slider, it goes from 1 to 39 (for some reason). and just like a few other options, the value defaults to 30.
however, if you use the console to set its value, you can go as high as you want. though if you set it too high, you will get a lot of "mouse drag". (or at least i did)
TLDR: what made mouse movement OK for me was:
"MaxFps" "60"
"inputrate" "60"
edit: polling rate only seems to make a difference if you have very high, or very low (<34 in my case) fps.