Can I clarify, you (were) in CBR, flying on a QF operated reward flight booked with AA points, presumably AA FF number entered on the booking, and, separately, you are QF LTS and CBR (was) limited by fog.
Are you asking if it would be better to check-in online before arriving at the airport, or hope the check-in / lounge staff will re-route you direct CBR-SYD in the ensuing fog related disruption for a shorter trip (not much point doing an ORC on a reward ticket since no points) and whether being already checked-in will affect their ability to do that?
My experience is that online checkin can be reversed (not checked in) but seat assignment is lost in the process (once all pax details were wiped, FF number, phone number, etc). This allows staff to flow forward, etc.
I have observed that staff willingness to assist varies depending on (guessing) checked bags, ticket type/fare (including airline/codeshare issues), elite status, pax attitude and phase of the moon
Personally, in this situation, i would arrive early, not checked-in with HLO and ask politely what QF could do to get me to SYD today, then relax and enjoy the lounge, reading and posting on AFF