I doubt that your get off was caused by adjusting the preload of your rear shock, although it may have been related... the biggest issue is your riding. Do you practice emergency braking? At 280lbs you're probably best set at 7, but your spring still wouldn't be enough for your weight. My understanding is that going down the preload at your weight from 7 would actually lower the rear ride height. By going down to 5 you made your suspension even worse, how this effects the rear coming up i'm not 100% sure, cue Defy or Ped.
The biggest problem with the scenario was not being aware of the situation (hard to know, but always suspect snobs (or anyone for that matter) will be an asshat, and pull dumb sh*t like that.) Next was over doing the front brake, too quickly. How fast were you going before you applied the brake and how much space did you have between said event? If it all happened last second, then there's not much you can do besides look out for it in the future. However, in the time you looked away, you may have covered the necessary space you needed to come to a stop. Quick stops/Panic braking/Emergency stopping should be something you practice regularly. As for trying to do an endo or stoppie, that really comes down to the rider and how he handles the brakes. I'm at 175lbs and I'm comfortable bringing up the rear when i'm bored... a friend of mine is probably 250-280lbs and can stoppie his SV650 no problem. It really comes down to you working the brakes.
Anyway, I'm sorry to hear about your accident, but more importantly glad you're relatively OK. Heal up, practice some quick stops and swerving, grow cynical about other drivers being idiots and not trusting them, and get back on the road soon.