I think I got Avidemux figured out. Today I edited and re-recorded three different versions of A Christmas Carol that I had DVD'd last week. Somewhat time-consuming, but it gave me a good excuse to do nothing but lie there and read Oliver Twist for a few hours while I kept a loose eye on the progress to make sure all went smoothly. The end result is not broadcast quality, but is plenty good enough for my own archives and is at least as good as the typical Taiwanese bootleg. After doing some more research on DVD file structure, I learned that the VOB files are actually the same internally as an mpeg, and don't have to be converted to some other format and then back again. So the process that was taking hours now takes only minutes. Also I learned that the video player that came packaged with my computer (InterVideo WinDVD 4.0) displays a much cleaner picture with mpegs than does Windows Media Player.
One of the versions was the Blackadder story, which is reduced to about 43 minutes without commercials. The other two were both two-hour TV movies, and both dropped down to less than 90 minutes without commercials. They each made an mpeg file of about 1 gig in size after editing. So that's around 35 minutes of commercials in a 120-minute movie. As someone else who once lamented the commerciality of Christmas said: Good grief.