Without preboot mode there's no way to recover your eTrex 10. One cause of most of the symptoms you've described might indeed be the wrong fw flashed to the device. However, as far as i know, apart from the US/EU HWID 1304 there are only JPN & SEA versions with HWIDs of 1472 and 1504 respectively and even in the unlikely event that your eTrex was one of those i believe that the basic hardware is the same as the 1304 device anyway. Therefore flashing either of those with US/EU Cure fw should not brick the device and the device should still boot sufficiently to stick on the Garmin logo splash screen but, importantly, with preboot still available.
A cause of all your symptoms can be a hardware failure, i.e. some physical damage to the flash chip which unfortunately can't be repaired by any software means. Usually when a chip fails enough both MSM and preboot mode are unattainable. Although it seems a little too coincidental that a hardware problem might surface around the same time you're trying to flash the device in fact it's often flashing it that's just "the straw that breaks the camel's back". The original problems of disappearing GPI files and the phone not properly accessing the device then asking you to re-format it might well have been due to the chip becoming unstable and therefore was never caused by bad or unreadable files. Subsequent flashing attempts (which stress the chip of course) may have exacerbated the problem to the point where the chip has failed further and so the device isn't capable of even getting far enough into the boot process to allow preboot. The reason i'm concerned about the chip is that a common software-bricking where bad files were loaded but the device shows the logo is always accompanied by preboot access.
Unless you can find preboot it's not recoverable and you may as well strip the screen, case etc to sell on eBay. So before you strip it for parts you should be absolutely certain it cannot access preboot.
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