I think b, a couple of weeks or 1 week after. I find when RP's start off with a big event straight off the cuff, it bottlenecks and then slows down to a crawl/people drop like flies (but maybe its just my crummy GM-ing) so when I do these kinda RP's I start with a steadier pace rather than big event ... but I haven't done a murder mystery so maybe it [i]would[/i] be really good to start BANG on the day/moment this all begins...

If you did start a week later you could already have the PD uncovering clues and whatnot though so thats worth considering ... 

Ohh, question, I aged up my dude to about 21, could he have been friends with the victim :) I noticed a sea of 17 year olds and ... well, he wouldn't fit in with that group and with no one else of similar age :S just wondering how to bring him into the fray.