Thats known and normal behaviour, the capsule-header got invalid after any mods and the flashing process will denied. You must use the recovery flashing method. I don't know. Perhaps the user is willing to test the mod. I have no programming skills, so it is sometimes try-and-error, which GUIDs have to be modded; there is no common rule, but if you have disassembling skills it should possible to produce a working mod with first try.
Yeah, you need to flash to the motherboard using test clips and a spi flasher . Fortunately ASROCK provides male pin headers for flashing. you just need one cheap ch340a type flasher or an Arduino or a raspberry pi to flash the bios.