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To see if it had something to do with file writing permissions, I launched the game as admin, slightly moved the slider and moved it back, relaunched normally (not as admin), and it seems the setting is now sticking as it should.  Hope this helps, and it's definitely a successful workaround for me.  I'll update here if anything changes.

BTW, I'm having fun with this.  Thanks for your hard work!

Despite the settings showing my modified mouse sensitivity, any time I start the game and go right into the mission it ignores the setting.  If I go into the settings to move the slider away from and back to its existing setting, it works again.