Having now resolved the downloads issue I've followed your guide here but alas during reboot it did not appear to do any firmware downgrade, so I am no further forward in finding a solution.
My only add to your list of symptoms and attempted repairs is that this appears to have been triggered by the surface pro 3 completely draining the battery during sleep mode and when we replaced the power supply and powered up it began with a failed boot cycle with a blue screen. Once we chose restart it booted normally except for the type cover that no longer works...
Really hoping someone has a solution for us - 2 hours later with MS phone tech and they have no ideas from their end..