I'm no expert here but a bit more troubleshooting should get you closer to solving the problem.
So do you feel pretty confident the blower motor is working and blowing continuously? You say air is blowing out of the vents, then it stops, and then just kicks back on randomly. When the air isn't blowing out of the vents, you should still be able to hear the blower motor working, if the problem was the blend door like you suspect.
Noted above says we have 3 servo driven doors in climate control, can you test a bit to see if you can narrow it down :
1. is for hot/cold temp adjustment. Excessive says this one is on the AC evaporator but the two below need dash removal
>>> So when the vents are working, does your hot / cold adjustment work?
2. is for location of air exiting, either upper vents, lower vents or combination of the two, and
>> Sounds like you suspect this one could be your problem. Could do some testing while its all working. Can you adjust the vent mode and does it work to blow from the upper and lower ?
3. is for recirculate or fresh air entry into the cabin.
>> Same same, does the recirc button work?