the first question is pretty simple they are like a humbucker but not a proper one, using a reverse pickup in middle is the nearest a strat is going to get to getting the humbucker sound (even though it sounds very different)
from wiki
"Many artists (including Eric Clapton, Jimi Hendrix, and Mark Knopfler) discovered that the pickup selector could be lodged in between the basic three settings for further tonal variety; since 1977, Stratocasters have been fitted with a five-way switch to make such switching more stable"
2nd question : you can make singles active with onboard preamp/eq other than that i dont know how.
theres a few on this page http://www.axesrus.com/axeknobs.htm