Can DE and FECN/BECN get set to "0" or "1" when transmitting frames?

Yes, on page 3 of 4 within the Frame Loss Test Setup, you can individually set the FECN, BECN, and DE bits to Yes (set to 1) or No (set to 0) within the header of your transmit frames. Then all of the frames that you transmit during the Frame Loss Test will be set accordingly.