I have no experience with cross-flashing EU fw to a US unit and until your post above I hadn't previously read a report of it being done so i cannot say for sure what the outcome will be i.e. it may not flash successfully (you may get 'system software missing' message, or it may hang on boot for instance), or if it does flash the unit may still not 'speak'. It will be at your own risk if you proceed following my uncertain suggestions, however the general principle with most Garmin units is that if you use the unit's original fw boot.bin combined with the other fw's fw_all.bin, then you should be able to successfully flash back to full original fw.

This is how i would attempt it: Do the general override of the HWID to 0265 in RGN_Tool. The HWID override is essential. I would also do the SW Version override to match the EU version, but i don't think that will matter. You might have to use the [Only registered and activated users can see links. ] to make it flash the modded gupdate.gcd, but try doing it normally at first. I would also use US v4.50 boot bin for the combined gcd, because if you have to flash back to full US that might make it easier to flash back using full V4.60 fw.

Good luck, and please let us know the outcome if you decide to attempt this.