For posterity dreamss, later on in the thread, provided the following alternative ini edits to disable mouse smoothing/acceleration:
is in APBGame\Config\DefaultInput.ini
+Bindings=(Name="LookUp",Command="Axis aLookUp AbsoluteAxis=0")
+Bindings=(Name="GBA_TurnLeft",Command="Axis aBaseX AbsoluteAxis=0")
+Bindings=(name="LookDown",Command="Axis aLookUp AbsoluteAxis=0")
+Bindings=(name="GBA_TurnRight",Command="Axis aBaseX AbsoluteAxis=0")
+Bindings=(Name="MouseX",Command="Count bXAxis | Axis aMouseX")
+Bindings=(Name="MouseY",Command="Count bYAxis | Axis aMouseY")
in APBGame\Config\APBUI.ini
bCaptureUnprocessedInput=False
in APBGame\Config\APBEngine.ini and APBGame\Config\APBCompat.ini and \APBGame\Config\APBCompat_Dev.ini
OneFrameThreadLag=False
I can confirm that only these changes are the only ones that seem to disable mouse smoothing for me. Arvie's linked in the OP do nothing for me.