Welcome!
1- Basic rule is if it doesn't say you can, you can't. (IIDSYCYC) So no lexan. (except in the cas of vents in side windows. I fabbed my own clips, I run a "Dry" install for easy swaps.
2-I'll let the BMW experts chime in on the best legal method of achieving your goal.
3-yes, if they are replacing struts. Camber plates are allowed. Again, the BMW gurus will have the best answer for the specifics.
4-internals only.

Drag chutes/paranoia. No, and yes! (you'll have no time if you discover a complete failure, and the effectiveness at our low speeds is highly doubtful anyway.)
Instrumentation. Consider a Data aq system. It will kill all birds with one stone from guages to lap timing to driver education. AIM units and the DL1 are popular options, but give serious consideration to whateve friends and racers around you use so that you can compare data.