If you've satisfied the DL MQD requirements or waiver, then you should still be able to buy some MQMs from DL, but it will be expensive, in fact very expensive for DM. Would you be better taking the rollover and trying to requalify for DM early this year? You'd lose a year of DM choice benefits but otherwise the difference could be minor depending on your travel patterns.

BTW whenever people have recently been asking about getting the AmEx MQMs, I always stress QUALIFYING spend posting by NYE in my response and your situation indicates that it's an important distinction that some people might not notice or be aware of. What did your AmEx spend show on the DL app as NYE approached? AFAIK those estimates include qualified spend only, although they're not updated according to any pattern these days.