I've got another autocross coming up on sunday and I'm excited since it's been almost 2 months since our last one. I love driving my 128i and the autocrossing it is a great way for me to push it hard but not worry too much about really breaking it. An old post of my early efforts:
http://www.1addicts.com/forums/showthread.php?t=250054
Since last years events I've ditched my run flats for 225 front and 235 rear dunlop star specs and that helped a ton, the understeer is still there but it's much more managable. The new tires are a lot more controllable when they break loose, I can feel it coming instead of just losing it with the run-craps.
My answers to your questions:
1. When I do "launch" mine from the line at an event I pull my rpms up to about 3500 and quickly get off the clutch, not dumping it because spinning out doesn't help anybody and I autocross with DTC and traction control off. It causes some awkwardness in the corners with the inside wheel wanting to spin more but I feel like I've gotten better at managing that than the electronics.
2. I probably shift into 2nd around 6000 rpms but can't say for sure since all the courses are different and I'm usually trying to avoid the cones. 2nd gear shifts I LOVE to take all the way to 6500... and there's something grand about taking it to 90 miles an hour in third, but I never leave 2nd at an autocross.
3. Join you local SCCA chapter and go autocross, it's the best fun you can have for $20 on a sunday.