Most of the newest Custom Firmwares have an option to access flash over USB in read and write mode. But dont know of any homebrew apps that allow this ( there is one with read mode over USB, but thats no help really)
You could try booting PSPFiller on it (put it under ms0/PSP/GAME/UPDATE/EBOOT.PB, as a non-kxploited eboot), as that can acess the flash and repair semi-bricked PSPs.
And the sellers explination of deleting files from devhook, wouldnt cause this, devhook is self contained on the MS, deleting it or any of its files from the MS, wouldnt of caused this, hes deleted something from Flash.
I cant say what files are damaged/missing, but you should look into dumping a copy of the 1.5 firmware ( or getting a friend to) And writting it to flash, if the Sony Update fails, there is a universal firmware flasher floating around somewhere (forget its name, or who wrote it), but it can write the offical firmware to flash without using Sonys installer.
btw, did you flash a version 1.0 index.dat via recovery before running the 1.5 update eboot?