FYI, the only time that I am aware that you can earn miles on DL with miles plus pay is when you have a DL AMEX credit card and choose online to redeem miles at 100 miles = 1 USD to purchase a ticket at an available published fare price. Buyers can purchase the entire fare with miles or choose a partial miles payment plus cash. Those redemptions provide full elite qualifying (MQM) credit plus requisite award miles for the fare. At times, the redemption can be quite attractive, although the goal of a frequent flyer is admittedly to obtain much better than a 1% redemption rate for miles. I have occasionally used that option to buy up to the first class fares, which includes the first class MQM bonuses.

Regarding DL's own Miles plus Cash options, my understanding is that they do not earn miles. On top of that, the redemption rates can be so bad, that it is advantageous just to redeem at the AMEX 1% rate which earns miles and MQM.