Mouse Sensitivity
The sensitivity is stored as a FloatProperty named BaseLookSensitivity inside:
%LOCALAPPDATA%\TimeSplittersRewind\Saved\SaveGames\options.sav

You can edit it via:
Hex editor > search BaseLookSensitivity > edit the float.
GVAS editor (e.g. uesave) > load options.sav > edit BaseLookSensitivity
Online editor (e.g. https://uesaveeditor.cc/): open options.sav, navigate: object  root > properties > baselooksensitivity_0 > set Float to desired sensitivity > save and replace options.sav.
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Mouse Smoothing
Mouse smoothing is enabled by default (bEnableMouseSmoothing=True).
To disable, open:
%LOCALAPPDATA%\TimeSplittersRewind\Saved\Config\WindowsNoEditor\Input.ini

Add the following section:
[/Script/Engine.InputSettings]
bEnableMouseSmoothing=False

Save and close.
Note: Mouse Acceleration is disabled by default
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CVars
To change hardcoded keybinds, use these as your reference to overwrite Input.ini:
https://pastebin.com/dVymr8wd
To force graphical settings (ignoring presets), use these as your reference to overwrite Engine.ini:
https://pastebin.com/SHHdmYGb
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Controller Sensitivity
Controller users can adjust camera sensitivity by overriding the right-stick axis configuration in Input.ini.
Open:
%LOCALAPPDATA%\TimeSplittersRewind\Saved\Config\WindowsNoEditor\Input.ini

Add the following section, treating the values as your defaults:
[/Script/Engine.InputSettings]
+AxisConfig=(AxisKeyName="Gamepad_RightX",
    AxisProperties=(DeadZone=0.000000,Sensitivity=1.000000,Exponent=1.700000,bInvert=False))
+AxisConfig=(AxisKeyName="Gamepad_RightY",
    AxisProperties=(DeadZone=0.000000,Sensitivity=1.000000,Exponent=1.700000,bInvert=False))

DeadZone: raise to solve stickdrift / ignore micro-movements.
Sensitivity: main viewsens multiplier.
Exponent: response curve (lower = more linear, higher = more acceleration near the edge).
bInvert: set to True to invert the axis.