BA has filed for slots as well. As the first departure from DUS has been filed for 0645, the idea apparently is to nightstop to aircraft in DUS rather than in LCY. The Darwin aircraft could indeed offer a morning departure, but according to the current slots filings, it would return to LCY in the evening. My understanding is that the Berlin operation is in co-operation with airberlin, so there would indeed be an oneworld link. However, a total of five daily flights to LCY seems to be quite a lot of capacity - my impression was that the DUS route has not attracted spectacular loads so far.
As the Darwin slots are still not cleared, it probably does not make too much sense to waste too much brain power on it.