I'm not going to say the yard did anything wrong launching your boat; because you were not there when it was launched. However; if I were the boat owner and the yard launched my boat without my permission or presence I'd be pissed. Maybe they do things differently in areas where you must haul during the winter; but I'd at least want a phone call and an ETA for launch on the day it comes up in the schedule.
How are they supposed to know if the boat is taking on water unless someone spends some time aboard immediately after it's launched? How are you supposed to know if they did anything to damage the keel while moving the boat?
You might consider talking to your insurance company; find out if they would cover your losses if the bolts are not failing due to corrosion or any other maintenance issue. If you remove the nut over that bolt that has 10 washers under it; you might find a threaded rod welded to the original bolt with failing threads. That would point towards failing keel bolts at the keel/hull joint.
On the issue of the location of the leak (inside the stringer); it should be a sealed bilge section there. There should be no holes to the keel/hull joint inside the stringer to allow water in even if the keel/hull joint has a gap. I suspect that since the core is disintegrating inside the stringer; bilge water got in there and was not removed before. Then it froze and expanded causing a crack to the bottom of the keel stub.
If the keel was 'alarmingly loose' on haul out; what was it's condition when they launched it? What was it's condition when it was hauled 18 months ago? You could not have caused this sort of problem by sailing it a few times (even if you sailed it hard). I suspect that the owner knew there were problems with the keel bolts; tried 3 times in the fall to sell it (holding off during the regular season so it would not be noticed for 8 months); and unfortunately you were the person who bought his problem.
I'm really surprised your surveyor did not mention the stacked washers on the keel bolt. That should be a dead giveaway that something is not right with the keel. If he is a SAMS or NAMS accredited surveyor; there is insurance they carry to cover them if they miss something.
You need to get the keel dropped and have the stub checked by a surveyor who knows naval architecture and can recommend a way of repairing the boat.
This yard launches probably 800 to 1000 boats a season and they want to go in the first two weeks of May, so there isn't much personal attention. There is a twenty four hour "float test" - in other words they launch at least 24 hours before your departure date. Apparently, I now know this must mean if it doesn't sink in 24 hours they assume it is good to go. The mast step was completed after it was launched and we had a TON of rain that day so I was not that surprised to find a large amount of water in the bilge as I had no doubt it was raining into the boat as they were working on this. The next day after our delivery it was apparent to me we had a problem.
I called the surveyor and mentioned the location and volume of the leak and he told me "boats leak" deal with it in the fall. Clearly unsound advice, which I obviously did not follow. Surprisingly his credentials were impeccable (Designed, Constructed and Operated a Research Submarine; Served 36 years in the United States Navy retired Captain of M.O.T.U-2 the Navy's inspection team for Submarines and Surface Ships; Twenty Year Member of Society of Naval Architects and Marine Engineers; etc, etc...) The boat was never surveyed in the water. I purchased it in November when all boats here are hauled out and could not do a sea trial on the boat.
The PO was ecstatic to see it sold and staying in Chicago and has asked to come sailing on it. He's the original owner of this 30 year old boat. I think if he had been trying to knowingly dump a money pit on someone he wouldn't have wanted to remain so close to it and he clearly still has a sentimental attachment to the boat, so I honestly believe this was a problem that he was unaware of.
I'm with you on the washers, they scream repair job - they are far enough aft in the bilge section that they can not be seen without unscrewing and pulling up a section of the floor - a task the surveyor did not do, although it takes about 10 seconds. Regardless, he didn't see them to inspect them.