well first of all you mentioned you wanna try this before you get a respray, so if your mindset is ok for a respray then nothing you do can make it any worse - have some confidence!!
on a flat panel, with a fast machine and G3, it takes about 5 minutes of polishing in the same place to polish all the paint off down to the primer, so dont worry toooooo much about going through, just take your time and be steady. if your still worried, then add some water to the panel at regular intervals as you polish - this breaks down the G3 and makes it less and less harsh (it breaks down the pyramid shaped cutting compounds that actually cut the surface back, and done properly can give you an almost swirl-free mirror finish).
your choice of 2000 paper is perfect, only removes a tiny amount of surface paint, and leaves the compound with alot less to do. the more work you do with 2000 the better. once the surface of the touch up paint is level with the origional paint, then get 2 small blobs of G3 (10p coin size each blob) on your mop head and spread it round an area roughly half a square meter in size. start the machine off slowly and build it up to about 4000rpm, making sure the head is always moving across the surface at about 2cm a second. if you leave the head in the same place for too long it will build up heat and start damaging the paint. when you see the polish start to dry out (after roughly 1-2 minutes on a fresh mop head) then add a bit more polish. soaking the mop head in polish DOES NOT make the job faster or look better, the mop head helps with the cutting and if you soak it in polish it becomes inneffective. keep going untill the flattening marks are mostly gone, then spray a small amount of water on the panel to thin the G3 and really get the shine back. on black you will probably get alot of swirls, these cant be removed by G3 unless your VERY good at polishing - so use a hand glaze to get rid of them.
failing all that - go and get it muddy!!