Based only what I have read online and here on this forum, I believe the CX-9 transfer case is spinning "all the time", and only the rear clutch engages in AWD mode.
So if dealer confirmed your 08 transfer case is deemed bad, it seems like your 08 has been in 2WD this whole time and the gears have not been spinning at all thus the reason for the clean oil.
Only way to confirm is drive it in snow, mud or sand and see if the rear wheels spin. Or hook up a GoPro camera underneath and focus on the long drive shaft that goes to the rear wheels, use white-out and paint on a white dot on the drive shaft and go for a drive, and see if that white dot is turning, if the drive shaft is not turning as you drive then your transfer case is bad, that drive shaft is suppose to turn all the time for a AWD CX-9 no matter what weather condition.
From what I gather, there's 2 ways for the transfer case to go bad, once the oil is bad and the gears seize, 1) the transmission splines get sheared or 2) the transfer case internal gears get sheared, in either case the car can still drive normally but only in 2WD, there's no power going to rear wheels. If your transmission splines are sheared, putting in a brand new transfer case won;t work, the transmission will just keep spinning without turning the transfer case gears.
Your 08 is still a good car, just not a AWD, there was a member here named Drogos, he test drove like 4 old used CX-9's in the
snow and 3 of them were bad, the used car dealer had no idea. That's why I think the reliability on the CX-9 is kind of skewed, FWD owners have good luck with the CX-9 but for AWD owners, it's a luck of the draw or they just don't know that the PTU has failed and it's really only a FWD.