HP released an advisory for the ZX40 workstations that goes over their advised method of crisis recovery. It is currently available at:
https://support.hp.com/us-en/document/c05163478
There is more to this than meets the eye, and the best advice is to read carefully to see if there are any tips you have missed. Then, also accept that at a point you may realize that there is no saving the motherboard and that it is time to give up and learn how to install a new/used one. I just ordered a used v2 one for a nice Z420 build that somehow I zapped recently. Rare, but it happens.