As for the .msu not many users know where to find it, or end downloading the wrong one :-(
Looks like it is not finishing the installation with the .msu either.
Also trying to install the .cab ends up with the same error0x800f0984. The only way that worked for me from .899 was Update in Place with ISO.
I think you may have misunderstood, yes you are correct using the msu file does not work until you do the in-place upgrade first.
There are many ways around this, with the common denominator being you have to get the device off of .899 before anything takes.
Perhaps your memory better than mine, but did we not go through this about a month or two ago with another "C" or "D" update as well, just don't recall the numbers. Not into digging through this forum to find it.
Welcome to the Insider program, this is what it is all about. MS may have fixed the update issue, but not from the faulty update once installed. You need to do an in-place upgrade via ISO. You retain all data.
Regards,