Firsy of all, go to
www.catalina30.com to learn all about the C30. And Join the association too!
Specifically, no the hull is not cored at all. Most all Catalinas are all solid glass.
Bolts do not stretch!
The "smile" is almost always due to improper blocking while ashore. The keel bottom actually tapers downward from front to rear and has quite a "rake" to it. Blocking must be as far forward as possible and a "nose" stand is desirable. However, the weight of the boat should never be on the stands.
The smile can occur either in front or back and is simply cosmetic. The overlaid glass and filler covering the joint can be broken by excessive flexing of the hull (accident or blocking). Most owners recover the joint when ad if it appears.
Only the worst cases of grounding the keel would result in movement of the bolts causing any leaks.