Is "sin" an informal version of "sinä"? I'm using a Finnish keyboard layout and hit enter rather than ä while trying to correct that word, and it was accepted without a typo warning.
I do believe that "sä" is the informal version of "sinä", and "sin" was probably just accepted as a typo of "sinä".
Yup. Although "sä" is something you might bump into in e.g. the capital region, and there are other alternatives. "Sun" would be the genetive of "sä" ("sun koira" - your dog (vs. formal Finnish "sinun koirasi")).