When are mac usually available for store pickup after an event ?
Also are they almost garanteed to go on backorder ?
In some cases they've been available directly after the event. We could certainly see an immediate release after the event or they may push the release to Friday to go along with the Nike Apple Watch and possibly the Airpods. Either way, I think its fairly safe to assume that the new Macs could be shipping to stores right now.
As for backorder, the supply will no doubt be low at launch but I don't think we'll see widespread sellouts. Apple has missed the back to school rush and the holiday season hasn't hit yet. Not to mention, the rate that most people update their laptops is much less frequent than a tablet or phone (especially for premium priced laptops). My guess is as long as your OK with buying a stock configuration (13" or 15"), you should be able to walk out with one on day 1 no problem.