SOMA,
I just finished fixing the same cracks on my Hobie 18. It took me about 3 weekends to do it myself and about $150 worth of fiberglass, epoxy, and parts. What I ended up doing to fix mine was drilling a 6" diameter hole with a Rotozip behind the rear crossbar and then laying up between 12-16 layers of fiberglass on the inside of the hull. I also layed up 4 layers of fiberglass under the lip. I put a 6" port hole where I drilled the hole. I was going to take it out for the first time today but the wind is dead here. Maybe next weekend. It was time consuming because even with an access hole it's still fairly awkward to fix the crack because of all the curves. It wasn't so difficult a repair that I wouldn't do it again though. The inner bolts on my crossbars were only torqued to 20 in-lbs when I removed them so I can't be of much help to you there.
Matt