Not an expert and only had a rinky-dink wet/dry, but:
1) I'm not sure how close you can get, but fwiw my bulkhead on a much smaller tank (8gal) is ~1/4" from the top trim and ~1" from the side trim.
2) I think so. This will be noisy by the way as air gets sucked into the line.
3) Per RC's drain diameter calc, you should be able to move 300gph with the return pump (after head loss) alright. Little less for saftey/redundancy would be wise, of course. http://reefcentral.com/calc/drain.ph...0&drain=Submit
No help with the bulkhead fitting on a round container. How about a square bucket?
4) I don't think it will affect the water line. A submersed pump halfway or at the bottom of the tank keeps the same water line (after accounting for etc variables), for example.