To answer your questions:
1) The smaller the rise time is for a PicoScope, the more vertical the edges and sharper the corners will be on a captured high frequency square wave. Bandwidth is not independent of Rise Time, and their relationship is expressed as:
Bandwidth = 0.35/Rise Time
Rise Time = 0.35/Bandwidth
where Rise Time is expressed in Seconds and Bandwidth in Hertz, or the equivalent multiple/derivative of the units (e.g. mS and kHz, uS and MHz, nS and GHz, etc).
There is a good article showing why this is true with a simple mathematical model here: http://www.interferencetechnology.com/r ... lications/
So, theoretically, the rise time and bandwidth are inversely proportional to each other. However, in practice they tend to relate, independently, to the criteria used when looking for a PicoScope for very specific purposes/applications. Bandwidth, i.e. the maximum signal frequency that can be captured tends to be important for considering the limits of analogue based signals (e.g. RF signals), while Rise Time is a criteria that is more useful when considering the limits of digital signals (e.g. clock signals and pulses).
2) The 10% and 90% references are the proportions of maximum height of the signal that represent the start and end of the rising edge measurement. The rising edge can also be specified as starting at 20% and finishing at 80%.