I can't speak for softcover or mhartl, but I made the jump to self publishing via leanpub for http://wewut . I haven't sold enough copies to make an impressive amount of money, but I've sold enough that I don't feel like writing the book was a complete waste of time.
1) I've found that supplementing material makes a massive difference in sales. However, when you see a larger percentage of the revenue, you definitely feel more inclined to create supplementing material
2) wewut isn't tied to fast moving software at all. I wouldn't consider that a necessity, though it does help ensure future editions
3) For some reason, self published materials seem to pop up less on piracy sites. Google a professionally published book and it's easy to find a free copy. The same hasn't been true for the leanpub books I've searched for.
4) Is it even worth the effort these days?
5) There's a bunch of stuff out there on this topic already - a quick search should get you plenty of pointers. Screencasts have been around awhile. Don't skimp on audio equipment has always been the #1 advice I've seen.
That's really great. An extra $35k is always a good thing. I published my second book (http://wewut.com) on http://leanpub.com, and I've been really happy with the results.