query helper methods and transactions

I haven't tested this myself but have you tried running insert_id() and affected_rows() before calling trans_complete()? I'd imagine that once trans_complete() is called, insert_id() and affected_rows() would no longer be tied to the queries run from within the transaction.

Yep, that's the most probable cause ... This happens either at the SQL level, or the PHP-extension one - CI has no control over it.