Hi tparris
Great to see that sharing info and experiences in this forum is paying dividends in resolving issues.
The serial number issue is something I have brought up with MS. They create a, Intune serial number on Kiosk and Full Managed devices. however they still interrogate the hardware for other details. They said there is one reason, some devices have 2 or 3 serial numbers. build serial numbers, stock serial numbers and the regular serial numbers. And therefore they could be reporting back the wrong one!!!! Doesn't make sense because all other types of enrolment appear to be able extract the correct serial number. I think this may change in the future, when Kiosk and Fully managed become more prevalent than they are already.
The enrolment side of things for KME Fully Managed devices is the only way I know. Works very well. There is a way to pre-load the user names within KME but I have not tried this as yet or should I say got it to work :-)