(*) Did you use the full unmodifed 3247 fw to flash it before and if so what was it named?

it sounds strange but i do not remember what exactly caused bricking - uploading wrong gupdate.gcd to garmin folder on device or flashing via update.exe
for flashing i was creating any random names, like test1 captain310 captain900 and etc by RGNTool from perrys collection of GCD
updater.exe was letting to do that. Ihave tried change HWID nad Version fields in RGNTool settings
it is crazy now to find how aps are sensitive to naming of file...

(*) Or did you use a modified or 'hybrid' fw of some kind with a different naming, e.g. <anyname>.rgn and if so what naming?

answered in first question

(*) Also, has the device actually booted fully since that flash? If it hasn't then you may be reading the XML reflecting its previous state - the GarminDevice.xml is only properly re-written on each successful boot cycle therefore it's important to know if it's been fully booted or not.

As far as i remember after firs attempt it did show gear symbol as per normal upgrade, but it never loaded to user interface, after reboot it cam to state with progress bar

(*) tried using gfw.exe to de-compile a GCD and re-compile the BINs to an RGN with safe-naming '3248' or '3246'?
and/or

i did use mbirths and gfw to decompile
but haven't found any tool to compile to new crypted format or RGN
I did try to compile bin files with RGNTool but got error that it has no clue what to do with file, region xxxx unknown and etc
flashing result rgn file goes super fast, just few ticks, so actualy it does not flash at all, but both update and webupdate says ok, it was successfull

(*) tried flashing with latest V2.8 [Only registered and activated users can see links. ] rather than using WU?

yes, I use the latest 2.8 as well latest WU too