Glad you got it fixed, $600 ain't too shabby either, I've heard of shops charging $1500 which is crazy because its not that big of a job.

Used car dealers suck, I've had my own issues with them in the past. I guess he doesn't think the HG is part of the engine. He may have even known it had a bad HG and put some stop leak in there which sometimes can fix the problem temporarily. When you had the timing belt done the coolant was flushed and maybe that broke up the stop leak and caused it to start leaking again. Your HG seemed to progress much faster than usual so I'm a little skeptical.

I bought my '05 EX new and love it, I'll drive it till the wheels fall off.