I lived there 4 a year, I wouldn't expect much at that time of year and its most likely not gonna blow anything worth the while.
Massive tides there though and when it's on its great.Just very hard to predict the weather there.
But on another note it has some of the best diving there in the world with amazing sea life and epic dives.If I was too go again the high would be the diving over the kite any day.
Sorry can't answer all your questions as don't have time, but here's to say you'll enjoy it.Best place for back packer mentality i.e party atmosphere is Nungwi Beach area out from stone town at the other end of the island.
Enjoy.