Ok, the problem could have started right there, forcing a downgrade to such a low version in one step. Please confirm if you did that downgrade by using Cure3 fw V3 with V7.8 original onboard which is what i think has happened.
Persistence is paramount when a device is not sticking on the logo screen but either 'bootlooping' or turning off spontaneously. In your case it's entering preboot as evidenced by the fact it's being recognized by Update.exe but because a flash isn't initiated quickly enough it's then exiting preboot and attempting to boot normally. To explain, when Updater detects the device in preboot (it only detects devices in preboot, or 'boot-block' as Garmin sometimes calls it for outdoor devices), if a flash isn't started in a short set period then the device exits preboot. For some modern devices that's a matter of mere seconds, many older devices had lots of preboot idle time like 30 seconds. However once a flash is initiated the device will remain in preboot provided the connection isn't broken or the flash's transmission of data packets interrupted.3. And then – everything went wrong. The Alpha 100 is now constantly in “Connect Mode” and does not enter the User Interface at all (except for the time that I forced it into Preboot Mode). The Updater intermittently recognizes it, but then drops the link. Sometimes, the “Connect Mode” forces itself over the “Preboot Mode”, but even without it, Updater wouldn’t work anymore.
So you still see it in Mass Storage Mode which is a good thing, regardless of whether it's been enabled by cure fw or not. See my final question below, no "trash" should be there after a complete successful reformat.4. The PC recognizes the device, and I can see the contents of its drive. There is a lot of “trash” piled up during time.
Well, i'm unsure if you mean you used V3 only initially and then 7.8 as asked above. It's important we know what it's running as it changes how we proceed whether it's got cure V3 or V7.8 or indeed even original of either version. Also it's contradictory to say "Updater manages to connect to the device, but fails to read it" because when it connects it is "reading it" by checking the device's ID info evidenced by displaying the info in the window next to 'USB Device'.5. Mistakenly updated via Garmin Express, and then mistakenly tried downgrading directly to Firmware 3.0 via Updater. When none of that worked, I formatted the unit using RMPrepUSB and then tried Updater again (after having GarminCure create ORIGINAL firmware). This is what I get – now, the Updater manages to connect to the device, but fails to read it.
Updater.exe uses Garmin USB Drivers to see devices in preboot so definitely no problem there.6. Garmin Drivers are up to date and functioning on other devices
What isn't clear to me is whether you have Mass Storage Mode (MSM) available because of Cure3 fw though.7. HWID: 006-B1562-00 (GarminDevice.xml accessible freely through Windows File Explorer)
I think not directly, however at some point the attempts to manipulate fw thru abrupt downgrade has caused problems. I suspect that you still have cure fw onboard which indeed intentionally 'bricks' the device because it's purpose is to stall the boot process from loading faulty files and allow MSM to be re-enabled so those files can be deleted or a re-format done. Perhaps you didn't need to load cure fw at all though because your problem is solely with firmware, not due to faulty or corrupt files at all.8. Tried GarminCure3, bricked the device
So again, i'm not sure of the timeline here. Is this still the state of the internal memory right now? You said that you re-formatted using RMPrepUSB however the dates of folders and files raise questions. GarminDevice.xml shows as 29/5/2020 for instance.9. Yes, Preboot Mode is possible, but the Memory can be accessed through Windows File Explorer also
This is the content of the Garmin Drive, from the backup (see image below)
Spoiler: pic
.........
I'm surmising that the device has serious firmware corruption but provided it's limited to the device's rgn14 [where fw_all.bin is flashed] then it should be recoverable. If there's problems resulting from a very low version of boot.bin used which in turn has poisoned either rgn5 or 43 or both it might not be so easy. As i said, how we now proceed depends on the answers to my questions above so please give a precise timeline as well as the actual occurrences.
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