Some ice in a tank parked in NC shouldn't be an issue. There probably isn't a low temperature low enough to cause catastrophic failure. Even if the tank was at Barrow,AK, the tank would have to be full to damage it. The piping, rather unknowable but I'd give the seller the benefit of the doubt. The tank sensing units are known not reliable. There are posts on this Forum, continually, about how to get them to read 100% accurate.
A leaking shower valve may be just that. If freezing water in the supply lines is an issue, you'll have bigger problems that that valve but there is no way to ascertain the shower valve leaks without running water through it and that should have turned up any other freeze damage. IOW, the valve leaks, not damaged by freeze.
