I think you have a good understanding of how such things work, so i'll give some more in-depth than usual info here to help you get your head around what we're facing to try to recover your watch.

Unfortunately, it's not possible to access or flash most regions in the device. That's because the firmware only contains the two flash-able components of fw_0505 and fw_all [for regions 5 (0x05) and 14 (0x0E) respectively] and it's not possible to 'dump' other regions because the device doesn't have media card capability. Therefore, there's no way to know what data is held in other flash regions and check their data against that of healthy regions, or other than 5 and 14, to know how many of the possible 255 regions even exist or are populated. I really don't think that's an issue however, i agree with your suspicion that the map update has caused this so that IMG file needs to be removed and it's held in either rgn48 or rgn83 which appears as the visible file system in Mass Storage Mode on a computer. Usually cure fw can re-enable MSM so even though we can't 'dump' that region will might still be able to access it depending on whether cure fw works.

However, in regard to using Cure fw, i just don't know. For sure QuickCure3 won't work because it uses ONLY a modified generic "boot.bin" (ramloader) which in a conventional Garmin device is flashed to a virtual region 12 (0x0C). That universal modified rgn12, when started stalls the boot process before any corrupt files, like maps are loaded - likely in the case of your watch it's the TopoActive IMG causing the soft-bricking. Because the process stalls the device can then be accessed in Mass Storage Mode to allow removal of the corrupt file(s) and fw_all in rgn14 hasn't been involved at all. Cure3 fw works differently, by modifying rgn14 data, which also stalls the boot process to allow MSM access.

Now if a 'normal' boot.bin's data is in rgn12, it will have written to region 5 (0x05) the bootloader aka u-boot and region 43 (0x2B) the x-loader. Those regions' data are further involved in the full boot process. It's part of the main firmware's job (fw_all.bin) in region 14 (0x0E) to first initiate the boot so if it's corrupt the device will try but cannot start and will auto-revert to preboot mode aka Garmin mode or recovery mode. Basically, the booting is a similar process to most ARM equipped devices, but in a "Garmin-ish" kind of way. So you can see that Cure3 fw (where fw_all data is modified), stalls the boot quite differently to QuickCure.

In the case of your watch and other devices which apparently don't have a region 12 for ramloader data, i'm unsure how the boot process works. Maybe they don't even have a rgn43, so no x-loader, and booting is handled exclusively by the bootloader/u-boot. Perhaps the main firmware in rgn14 is responsible for starting only rgn5 directly, or it searches for something else initially. I really have no clue about that, sorry.

If you want to try it, you can try making a 'cure' version of fw_all.bin only. I know for sure that Garmin.Cure3.exe won't handle the watch's full firmware file because of the unusual fw layout - it would be expecting to see rgn12 data in the first section, not rgn5. But if the fw_all component is the same as that in conventional devices, you might be able to modify fw_all,bin properly so you can then re-combine it with fw_0505.bin to make a full cure RGN that is flash-able to the watch. If it was my device i'd try that anyway. You may have to use a different tool to recombine the fw, such as [Only registered and activated users can see links. ] because RGN_Tool will probably not work for that. Also, if Cure3 won't process the fw_all file either, you might be able to make a cure fw the old-fashioned way by changing file names in a hex editor. In your case, search for any instances of IMG and change them to XMG or something that doesn't correspond to any other extension used in the fw. See here: [Only registered and activated users can see links. ].

Even if it kills it by flashing a modifed fw, you'll be no worse off because Garmin won't repair it and I don't know of anyone outside Garmin who can do it. Might be worth first searching around though. Good luck. If you need any further help with the fw manipulation, just ask. Still be at your risk solely of course even if i make files for you.