You need to contact SAA for seat assignments - it is their flight. Anyway, just do OLCI at 24h before depart. and select two seats next to each other. Shouldn't be an issue at all. Even if it fails the last resort will be to ask the pax next to you to swap seats with your partner (or ask the pax next to your partner to swap with you) - usually pax don't mind in particular if you offer a someone in a middle seat your aisle or window seat .
To merge the tickets is not possible.
Within Star Alliance it is always the operating carrier that determines the FFP earning - the ticket issuer is irrelevant. So you need to look up the Asiana FFP earnings for flights with SAA. Hopefully, your LH T class corresponds to SAA T class and not some other non-/low earning SAA booking class (I don't know this, but most likely it will but there is no guarantee).
Last edited by SK AAR; Sep 16, 2019 at 5:08 am