1) In Ohio you should bury them at least 3 feet to avoid freezing in the winter. If code requires deeper, go deeper.
2) As long as you do not plan to drive over the pipe, it is OK to backfill with soil, so long as the soil does not have any sharp pieces of rock in it. Use a heavy tamper on each six inch lift. You can make your own tamper by putting a 20 lb iron or steel plate on the end of a wooden dowel. If you plan to drive over the pipe, you will need to use structural fill (i.e. gravel or crushed stone) around the pipe. I recommend using Schedule 40 PVC, much tougher than Schedule 20.
3) Usually people put a plastic or better yet a concrete apron at the outlet. Check with your local code official, there is probably a standard in your town.
4) You will get only a 1/2 inch of settlement per foot of soil or so if you tamp it carefully. If you do not tamp it at all, you could get up to 25 percent settlement, depending on the type of soil you have.
5) The only way to know how much money you save is to get a couple of quotes from local contractors, then decide how much you value your time at. How were you planning to excavate the trench to the pond? If by hand, add some cost for the hospitalization charge for the bad back you will end up with (if you are over 40).
By the way, you may need a permit from the town for this work, I strongly recommend you check first. Also, be sure you call Dig Safe before you start digging, if you hit a gas line or buried electric line it could be a really bad day for you.