Problem solved: (Memcard|OnboardMU):\\ExtendedSystemPartition was missing due to taking shortcuts with the dash upgrades.
This was the solution:
- restore stock 8955
- update to stock 14699 via usb
- E80 -> update successful
- make a retail 14699 with 360 multi builder 0.92, with corrected cfldv
- flash retail_flash.bin
- boot into dash
- format the OnboardMU
- do a new 64mb dump
- run 360 multi builder 0.94 on the new dump
- flash updflash.bin
- fsd on (6750 zp / 8453 / 14719)
Note: all flashing and dumping was made by nandpro, since I didn't need xell due to already having the cpukey.
So, in short, the issue was a missing ExtendedSystemPartition due to not updating to stock kinect dash before RGH.
Regarding bad data in cpukey.txt: it was caused by notepad prepending a UTF-8-encoded UTF-16 BOM (U+FEFF).