1. 3-4 days per week. 16 miles each way, so I'm on the low side for this question.
2. Yes.
3. I always have done it under my own power, but I've been a commuter/recreational rider my whole life. I've been commuting on bike since 1998. My time has come way down since I started riding more frequently, probably down from 1:15 to 1 on the way in and 0:48 to 0:56 on the way home.
4. I've commuted on an old Bridgestone (then sold), a 1999 Schwinn Mesa (then stolen), a replacement old Raleigh MTB (not so good --- I donated it in the end), and my present bikes a Kona Jake and a Fuji Sports 12. I've only done the 16 mile commute on the Kona and Fuji. Overall, it took a while to get my seat to the right height. I don't think that any other modifications are necessary other than that. My attention to maintenance, including tire pressure, has gone way up because I don't want to be stuck in the middle of a 16 mile commute.