Ok, so now we know some certainties about the device. Importantly it's UID has not been permanently lost and it's now got HWID of 2512 also with ramloader of O750t flashed to region 12. We also know (i) either nonvol region is not 41 but 154; or (ii) that the path to card isn't 1. So maybe the card is 2 and nonvol is in region 154, or .... some other combination. So let's test it with other combinations and also try to get a dump of GarminDevice.xml and gupdate.gcd.

Remove last_id.bin from the 2512 folder and replace the contents of update.txt with these commands then try again:
Spoiler:
rrgn,154,1:/154.bin
copy,0:/Garmin/GarminDevice.xml/,1:/
copy,0:/.System/gupdate.gcd,1:/
rrgn,41,2:/41.bin
rrgn,154,2:/154.bin
copy,0:/GarminGarminDevice.xml/,2:/
copy,0:/.System/gupdate.gcd,2:/
reboot

After you copy/paste the text ensure that you haven't accidently created any additional spaces behind lines or a 'carriage return' anywhere before using. Post all results including the new entries in update.log