I think the most reasonable and probably outcome is for the DOT to defer the DL/UA application and grant AA dormancy for one year. If AA does not resume service within the prescribed timeline, these frequency can still be assigned to UA/DL for use in 2020. Considering both UA and DL received dormancy waiver for China frequencies in the past (some much longer than AA is requesting), and no other airlines has been given an 365+ days start up time, it will be unwise for DOT to set such a precedence and unfair to AA given their lack of onward China partner until recently.
I have to disagree with your post, although UA was granted a dormancy waiver for some time, when that waiver was received there were frequencies available which were going unused. Also if I'm not mistaken at the the time the dormancy waiver was issued neither DL or AA was interested in increasing their U.S. - China frequencies.
Also when UA announced intentions to go daily double they state the reason the service wouldn't start till 2020 is because they need time to negotiate with China in order to secure a favorable slot. There is no need to negotiate with China for a landing and take off slot if the DOT doesn't award you a frequency. And as we all saw with AA's ORD-PEK frequency at first they delayed the start up of the flight do to a less than favorable slot then for a few years they were forced to fly the route with that less than favorable arrival slot. In the end they were able to negotiate for a better arrival time slot but it wasn't what they wanted which was an early or mid afternoon arrival time slot. I'm not sure how many people in the past actually paid special attention to the negotiations airlines have with China when they add service to crowded airports like PEK and PVG, it has been notated many times on this site just crowed the skies over PVG and PEK have become.
Personally I think AA should be given till June 2019 to put these flights back on the schedule, service to begin in October or November 2019. When June 2019 comes around if AA has not announced firm concrete plans to resume service utilizing these frequencies then I think the DOT should at that point award should award DL, MSP-PVG; UA, EWR-PVG 2X daily, and give AA a dormancy waiver for one of their unused frequencies and that dormancy waiver should expire in November 2020. If they aren't using that frequency come November 2020 at that point it should be returned to the DOT if no other airline has submitted an application to utilize the remaining frequency.
Awarding DL and UA the frequencies in June or July of 2019 gives these airlines in my opinion enough time to negotiate with China for the time slot they believe works for their customers.
Also one last thing if AA truly believes the tide will change if they can cultivate a closer relationship with China Southern then I think they should fly the routes now, even though the routes are loosing money. If AA believes they are close to developing closer ties with China Southern which would benefit them and their customers why cancel these routes when you know how highly valuable these frequencies are especially if you are so close to a deal with China Southern? I'm not sure if AA's rebuttal helps or hurts their case for a dormancy waiver.