1) I analyse my files mainly during ripping, otherwise when I remember. When I first started off, analysing my 3,000+ tracks, I created a smartlist to limit the number of tracks being analysed to 100 (ie: playlist="Task -- Not yet analysed" ~t=100). Then I did them whenever I could.
2) Fixed. I use it in a restaruant /bar environment and using the "based on playlist" setting can sometimes jump the volume when you change playlist -- and usually the wrong way. Ie: if it's time for some rowdy music, MC will generally play the rowdy playlist a little softer than the "chilled" playlist. So I use a fixed value of about 6dB.
3) As far as I'm aware: BPM is obviously beats per minute (or an approximation thereof) and intensity is a measure of the "loudness" of the music. ie: a rock metal band will be more intense than a Mozart string quartet. I make smartlists in conjunction with genre. Ie: for a chilled atmosphere, my smartlist says "genere=house bpm=<120 intensity=1,2" and for a lively party, my smartlist says "genre=pop,rock bpm=>100 intensity=4,5".
Also, my mother is an aerobics instructor and it's very useful being able to use view schemes to group similarly tempo'd music. ie: for benches you need music between 60 and 70 (actually, I have no idea what music you need, but it's an example anyway!). So I've set up a view scheme which allows her to see what music fits into each category and then make her tapes / burn CD's from there.