i got another case and some proper name brand tool bits for the cheap knock off dremel tool (especially the diamond cutting disc made a huge difference) and very slowly and deliberately planned my approach cut by cut, then proceeded to smooth the edges with a grinding bit and finally polished the cuts with reeeeaaaaaally fine sanding paper. the screen cut-out i did with an x-acto knife, that gave me more control to leave an edge which i can glue the screen protector to later. it's by no means perfect but at least i still have the bottom screw holes and no visible blemishes on the outside. for me what worked best is putting the dremel to maximum rotation but doing short and precise cuts so the plastic doesnt get to hot and starts burning. melting is fine, smoking, burning and discoloring is no-no's.
the button holes i did with a step drill bit but since i neither have a drill press nor a proper torque wrench to hold the bit i actually twisted it into the case by hand, bit by bit. the result was 2 surprisingly good looking button holes and three bloody cuts on my fingers from gripping the drill bit so hard.
so as you can see, i'm very noob to all this myself, the most important thing i learned so far is "take it slow, measure twice, cut once"
here's some pics:
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