"but it does look like the entire account was sold off."
Having the entire account sold off does make things easier. If you can come up with a total basis it doesn't matter how you slice and dice it among securities; the net gain or loss will always be the same.
"Would it just be the principle that was added to the account since it was created?"
No, not really. Any "reinvestments" - dividends, LTCG distributions, STCG distributions - would add to basis. More problematic are "corporate actions", particularly mergers and acquisitions, that could affect basis DEPENDING ON HOW DAD reported them, and these things often get reported incorrectly. All you can do is plow through that information and do your best..
