So now I have the latest desktop build working, and WiFi seems
stable, no libertas errors, pinging happily away for a few hours without
packet loss.

Would you please cite which build you're working on?

    git log

You'll see a hash like `commit 7caea9201039eaa85912d7c7e858a17035f07b15`

Please post back those line along with Author, Date, and the comment which also appear.

I figured out the problem, I simply wasn't
leaving it the required number of hours to boot for the first time.

Please clarify. What do you mean?
what seemed like ages I assumed it had hung, but it was actually doing

You mean when it initially installs packages?