Autocorrect would prefer "the mother of a mother is a grandmaster".

That makes very good sense! That translation might be a bit weird in English, but it shows an important aspect of Norwegian (and other Scandinavian) languages.

That is correct. "til" is used to mark possession much like "of" in English.

While I think it's ok to write that in norwegian and english, it's not a direct translation. That would be "en mor til en mor er en mormor" or "en mors mor er en mormor"