Yes just like your outdoor garden hose valves ice is a powerful force able to split most metal...I could not being to advise you on the keel
stub and surrounding hull
If 18" froze solid i would imagine there would be some stress damage somewhere...how bad or big of a concern I think is going to be an individual case by case issue.
A competent surveyor
will know what to look for if you do not...but you yourself can tap around listening for delamination
....that is going to be the surveyors main mode of operandi along with looking for the obvious...cracking, tabbing broken loose..and of course rotted bulkheads and and structural glassed in wood stringers that were submerged for that long.
A lot of us here don't put a lot of stock into but very few surveyors, as we have learned they are notorious for missing things anyway...after all they are not there to do destructive testing or spend a week doing it...Your paying them for say half a day to give you their opinion based on there past experience...that's not a lot of time to crawl around a boat and expect them to find everything.
Learn what to look for and you pretty much can do this yourself...Your Insurance
co will think differently if going for Comprehensive/Hull insurance
Ask around for a through surveyor
and have him take a look if you don't feel comfortable yourself....no offence but your questions tell me you would be better off with a surveyor till you get up to speed ...follow him around and ask questions you will learn a ton.
PS if your in SD hire Kells Christian...he does one heck of a through job.