Now you are on a different topic, so let's finish the previous one first. The 8-bit string "ä" is utf-8 encoded as "ä", so if string_to_utf8 was altered it to not encode "ä" it would perform incorrectly.
Over to SQLite. The SQLite interface takes utf-8 encoded strings. So if you pass a utf8-encoded string to big_query it will be encoded a second time only to be decoded directly on the receiving side and be stored as the origianl utf-8 string in the databse. When another application reads the string it will again be utf-8 encoded by sqlite and returned to the user utf8-encoded twice. If that is not what you want, don't utf-8 encode the string. Note that for the %s API, 8-bit strings are stored as BLOBs unencoded, but they will still be utf-8 encoded if read as text.