sorry to hear that, not heard of many DOA axolotis, to have two is really unlucky.
the one that is asking constantly for firmware updates, you should try to do the manual firmware (rescue) update, details are in the user-guide section.. its sounds like the normal firmware update is not actually successfully doing the update. I've had that before and the rescue mode fixed it.
generally, you need to PM @Johannes to discuss , click on this link to see his profile, and use message. (ive raised your user to next level, so messaging is now available)
provide him with as many details as possible, KS email, pictures of boards, details of what you've tried so far e.g. different usb cables.
good luck, hope you get it fixed asap.