yes the audio can be disabled so its nto recording. you can set it to not record audio by default.
you cant actually turn the gps off, you CAN turn the speed display off though. and you could easily get rid off any gps data by converting it in video software. i have mine set to not display speed just for the same reason as you, i dont speed much anyways, so little differences in speed I would argue is not precise in determining with video.
it uses a capacitor. it also will shut off if it gets too hot, ive had that happen several times with my 550 in the rear window because nothing is blocking it currently from the sun, it is directly in the sunlight so when i go to work and park for 8-12 hours it can get too hot. i have been meaning to use something to block the sunlight though.