is it possible to lower the mouse sensitivity for FFvTTR?
.. for my mouse its _much_ higher than on the desktop.

there's obviously no option for this in-game, but maybe there's a way via altering an "ini"-type file, or something?

Well, I have a Wacom tablet with a cordless mouse. I cannot  play either Freedom Force games with it, because it jumps all over the place. I hooked a regular mouse up to the computer (Tablet is USB), and the regular mouse worked fine. I now have a Logitech cordless mouse, and it works fine. No go with the Wacom yet.
That probably doesn't really help you, except to point out the possibility that it's the mouse. The Wacom has the capability of setting up some different settings for different programs (Photoshop, Word, Illustrator). You might check and see if that is available for you.