Unless you are dealing with a proper travel agent, old school type, then I suspect you will indeed be stuck on the upgrade issue.

You should be able to reserve a seat, however. I'd thought if you leave it a day or two it should come up as an online option. You may want to play around with the Finnair website to see if your Air Berlin FF number has truly been attached to the booking.

You can also upgrade a travel agent ticket with Avios in certain circumstances, however from these details I bet that won't be possible - it needs to be in Y, B or H buckets and I very much doubt that is the case. You won't be able to do a cash upgrade in advance except via the agency.

However if you check in online on the dot of T-24 you should be able to avoid the middle seat syndrome. If that fails, ask about an Airport Upgrade Promotion at the airport to buy your way out of trouble. Usually only one class up is allowed (WTP) but you may get lucky.