I can't comment much on the .308 - it's a great cartridge with accuracy potential beyond the range of most of us mere morals. However (thankfully?) your problem isn't limited to just .308's...
It's something I am dealing with right now with my Rem 673 .350RM Bolt gun. It's a true short-action (and considered to be the "original" short mag dating back near 30 years now). The 350 is limited to a 2.800" max COL. I can tell you unequivocably that 2.900 COL's will NOT fit into the mag well at all. 2.800 is it - the max.
This round was designed to handle a 200gr pointed bullet over around 55-62 grains of powder (4895 or so). 200 gr bullets normally seat right at the cannelure.
This short 2.800" COL poses several problems, not the least of which are cannelures and 250 grain bullets. Both the 250 Speer and 250 Hornady bullets have cannelures that ultimately get burried deep within the case just to be able to stick to that 2.800" max COL.
So, in addressing one of your questions: Don't worry much about the cannelure and where it should be seated. If you are getting great accuracy at the speed/power you want, who cares where the cannelure is!
As to the question of whether or not COL affects accuracy, it absolutely does! A couple hundreths of an inch can make a measurable impact on a group. Some people argue that they want the "jump" that the bullet makes from the case to the rifling to be as short or non-existant as possible. Dan Newberry makes quite a commentary on this and I highly recommend that you take a look at his webpage on that:
I'm not going to bother to try to parapharse his work. there is a lot of good writing in there and it's well worth the read. Long story short, Dan suggests that one find a stable powder weight/bullet combo and then by varying the bullet seating depth one can further affect accuracy.
Anyways, welcome to the wider wonderful world of reloading!