it sounds more a hardware issue than a BIOS issue, unless there were other working version of BIOS.
it will be reason enough if you have gone thru all troubleshooting steps.
just be sure to add the links to the post and inform them all the troubleshooting step you have gone thru.
that way you can speed things up.
so RMA will get you a working motherboard at least. and you can sell them if you decide to go for other motherboard.
lemon happens,
the first 1700x i picked up, had **** memory controller that it wont go beyond 3000Mhz, swap and easy 3200Mhz
or just like the 2700x that I had just swap out was crap, way below avg performance.
totally understand the frustration that you have, just make sure you explain the issue and ask them nicely.
as long as it's under warranty, they should replace it for you.