Well, Banggood sent a replacement. I'm happy to say this one works fine.
Second unit's price was 120€. Shipping it to China was more than half that amount, so this time Banggood didn't ask me to return the unit to the OEM. They just sent the replacement to me instead. I'm quite happy with how Banggood managed it all. It could be way faster, however, and when they pay the freight expenses back to you, you lose PayPal's slice (if you paid via PayPal, that's it, because they use the way you choosed for payment in the first place). Should you need to return something with great expense, this could matter. But in this case, these have been just minor nuisances.
On the second, damaged unit, I have been able to fix the fan. I have to say I just got the aluminum plate out and sticked it again with some epoxy. It was sitting perfectly in his place. Also I didn't noticed any place where there could be friction between the blades (aluminum plate included) and any other parts, yet there was friction somewhere even after opening the fan. If made to spin with a finger, blades stopped almost immediately.
I got the aluminum disk out just as a first step, hoping to find where the problem really was. But then I was unable to find any way to dismantle it more without destroying it. Neither from the blades side nor from the motor side. After undoing the motor body, it seems there could be a couple screws/bolts, but they would be quite small and difficult to reach. Then I placed motor body again. Then I noticed the blades remained spinning longer after being finger-impulsed.
So I placed the aluminim disk on again. I needed to file the plastic welds since they have mushroom shape. I just made them a cylinder. Hammered the aluminum disk to get it straight again. Then I placed a little bit of epoxy in all plastic surfaces that would be having contact with the aluminum disk, and clamped it all while epoxy was curing. A couple hours after that, it was working.
Don't ask me what the problem really was, I don't know. I didn't noticed anything weird while dismantling, friction aside. Perhaps blades were misaligned and the force used to get the aluminum disk aligned it? Or perhaps it was something in the axle? Fact is it seems to be working fine now. I'm taking it gladly anyway. Not bad to have a little bit of good luck after receiving two DOA units in a row.
So I could now just buy the heater (it melted when trying to give a second video to Banggood) then I would have two working stations... or could I let it as spares source? I don't know, will see after I put random_hack's board into the new unit.