To back up an existing firmware use LtnFW.
The procedure you quote for cross flashing is correct, at least that's what I did.
Yes flashing back to a 2400 should be possible using LtnFW. I say should as I've not done it & I doubt many others have.
You'll probably get away with the RMA having done all this if you have no moral objections to it. Though personally if I'd screwed it up I'd just bin it.