Hello, here are my answers.
When I say fails to enumerate I mean USB fails to enumerate, no RT1050 drive shows up and dmesg shows no USB activity whatsoever. And because USB doesn't enumerate, MCUXpresso can't see the probe at all. I have tried Ubuntu 14.x (a VM) and 16.x (a PC). The 16.x behavior is somewhat better in that it occasionally enumerates USB.
The DETAILS.TXT file is long, sordid tale. If you go to to the OpenSDA debug adapter site you can download two firmware files, k20dx_mimxrt1050_evk_qspi_if_crc.bin and k20dx_mimxrt1050_evk_hyper_if_crc.bin. Bizarrely, it is k20dx_mimxrt1050_evk_qspi_if_crc.bin that lets me program HyperFlash on the Rev. A3 boards (?!) and I have confirmed this repeatedly. Surely the files are mislabeled, if not then please carefully explain this naming convention. Anyway, here is what the DETAILS.TXT contain for each FW/Rev variant.
DETAILS.TXT - Rev A4 - k20dx_mimxrt1050_evk_hyper_if_crc.bin
# DAPLink Firmware - see https://mbed.com/daplink
Unique ID: 0227000041114e45003d300dc40300105ab1000097969900
HIC ID: 97969900
Auto Reset: 0
Automation allowed: 0
Overflow detection: 0
Daplink Mode: Interface
Interface Version: 0244
Bootloader Version: 0242
Git SHA: 853df431d81359e822f49363891f877f17d31efb
Local Mods: 0
USB Interfaces: MSD, CDC, HID
Bootloader CRC: 0x73707d49
Interface CRC: 0x565000e7 (TAKE NOTE!)
Remount count: 0
DETAILS.TXT - Rev A4 - k20dx_mimxrt1050_evk_qspi_if_crc.bin
# DAPLink Firmware - see https://mbed.com/daplink
Unique ID: 0227000041114e45003d300dc40300105ab1000097969900
HIC ID: 97969900
Auto Reset: 0
Automation allowed: 0
Overflow detection: 0
Daplink Mode: Interface
Interface Version: 0244
Bootloader Version: 0242
Git SHA: 853df431d81359e822f49363891f877f17d31efb
Local Mods: 0
USB Interfaces: MSD, CDC, HID
Bootloader CRC: 0x73707d49
Interface CRC: 0x16f4aa60 (TAKE NOTE!)
Remount count: 0
DETAILS.TXT - Rev A3 - k20dx_mimxrt1050_evk_qspi_if_crc.bin (BUT USING HYPERFLASH?! SURELY THE FILES ARE MISLABELED!)
# DAPLink Firmware - see https://mbed.com/daplink
Unique ID: 0227000041114e450025300ac207002a92d1000097969900
HIC ID: 97969900
Auto Reset: 0
Automation allowed: 0
Overflow detection: 0
Daplink Mode: Interface
Interface Version: 0244
Bootloader Version: 0242
Git SHA: 853df431d81359e822f49363891f877f17d31efb
Local Mods: 0
USB Interfaces: MSD, CDC, HID
Bootloader CRC: 0x73707d49
Interface CRC: 0x16f4aa60 (TAKE NOTE!)
Remount count: 0
Regarding your questions 3 and 4. Yes, I have done all of this over and over and have been careful to follow each step, DIP switches and all. As I mentioned, the flash erasing process worked and had some positive effect but the A4 probes are still unstable, giving the messages I pasted in the previous response.
The behavioral differences in the A4 boards are not subtle and the problems exist across Linux, OSX and Windows. There is clearly something different about them. The only HW change in the schematic is "Update BOM only to change CSI related serial 0ohm resistor from DNP to mount, mount R352 and DNP R353". On a whim, I've tried reverting this change to no avail.
Regards