i would not drill for the return, you risk emptying to the bottom of the hole should your power go off. this could be quite a bit of water even at 1" below the water line. use plumbing up and over instead, that way you can alter later on if you want to add, say a sea swirl or some other rotating device. you can use non-return valves etc but they can get clogged and reduce flow. only drill for closed loops.
here is my return into the 1" sea swirl:
any drilling will weaken the glass, he's covering his arse by telling you that it will be strong enough so long as any drilling done is not too close to the edges.
for the overflow hole, drill it near the bottom, but as above, not too close for obvious reasons. then, add a durso standpipe to the mix to cut down overflow noise. heres the best pics I could find of my overflow: