My own recent experience, arriving at 00:35am in business, when getting off the plane was directed to the PCR testing area, wait time was approx 15 mins, just a quick doc check and contact details taken. Results were available on the app about 6 hours later, and get a text around same time. I got to my hotel in the Marina around 1:45am, slept for a few hours and woke up to the results, so the need to wait in the hotel for results was irrelevant. I dont think Marhaba services would have made this any faster, but then i am very familiary with DXB and the airport set up there, so perhaps you would feel happier with assistance, as it is cheap anyway

With regards testing in Dubai before your return to USA, there are plentiful places around dubai for cheap testing and should cost no more than 150dhs, but i would highly recommend using Al Tadawai home visit for 240dhs per person. The service is so quick and convenient as they will come to your hotel at a time that suits you