Just make sure if you vote in person (early or day-of) that you bring your absentee ballot with you. They'll void the absentee ballot and then you'll get a new ballot to vote. If you don't bring your absentee ballot, you vote provisionally so that they can make sure your absentee ballot doesn't also come through (i.e., make sure you only get one vote).

The witness thing is easy enough if you know another human being, though. I didn't like it at first, but it's just a signature, not a huge deal.