====== XFCE Wifi remains disabled ======
  # rfkill unblock all

That makes sure it's not blocked.

Then the order of steps is important:

  # systemctl stop NetworkManager.service

  # nano /var/lib/NetworkManager/NetworkManager.state
    change from false to true on line "WirelessEnabled=true". Then:

  # systemctl start NetworkManager.service

Then everything goes back to normal and wifi works. It seems network manager writes this state as false when it stops, but reads it when it starts, so a simple restart isn't enough.

In addition, I noticed that sometimes, despite these steps, I also needed to kill and restart nm-applet to get the options to not be grey.

  $ pkill nm-applet
  $ nm-applet &
  $ disown