No need to flash bios for mining (just if you want to squeeze last 0,x MH from it, in my opinion doesn't worth the risk), also if the card is dead, bios is useless. I don't think they desolder bios for checking it if was flashed or not, also if they do that, you can blame them for flashed intentionately that bios/or hardware modiification especially for voiding warranty. Is your word against they word. I don't know they are accreditate to desolder bios chip or make hardware modification (as seller or service, it they are not official AMD), any hardware modification will void automatically warranty. I think important is to have intact AMD sticker on screew.