I contacted Delta via Facebook and they were able to reinstate the AF flight, however issuing it as AF0007 instead of DL1015. Also they were not able to give me the original seat assignments free of charge. Luckily the FB hotline put me back in exit row of the upper deck for free even though I am not Platinum anymore (used to be at time of booking).

Will I get less miles now that part of the itinerary is on AF numbers instead of DL? Out of curiosity, how will AF determine the ticket price to take as a basis for the miles calculation since DL sold me the ticket?

All in all an unsatisfactory and totally unnecessary experience caused by Delta.