I do hope this thread continues
I find the whole incident fascinating, and avoidable. One of the great assets of a forum is the ability to share information and suggestions to avoid a repeat scenario. A couple of points from my dim dark past:
Unless the crew is supplemented, at some stage in a long sector a pilot will leave to go to the toilet. The pilot remaining will then be single pilot, obviously. To those who say that 2 pilots should be on the flight deck for a level change, I question what happens when ATS say "Due crossing traffic climb NOW to F360"? Do you reply "Unable due captain having a leak", and then accept the 90 degree turn? What does the captain think when they feel the aircraft change heading in that scenario? Personally I would cease "splashing the boots" and get back into the cockpit and ask "What were you thinking?!!"
If an F/O is checked to line, then they should be able to handle any situation when the captain is not present, from TCAS to level 3 warnings - if not then they should not have been checked to line. From past experience the limitations of the Airbus autopilots are not widely taught, but the microchips and technology date back to 1980. In those days your computer would have been a Commodore 64, TRS-80, or if you were flash a Sinclair Z-80! These same chips power Airbus A320s today, and the autopilots are woeful. If ATS gives you a speed on descent of, say, 320 knots - the autopilot will handle that plus or minus 20 kts!! Reportable speed violations!
Next, an SOP from a long-defunct airline. When a pilot leaves the flight-deck, a flight attendant comes in, if only to open the door to the pilot outside in the event of incapacitation. The F/A cannot obviously occupy a control seat, so stands or jump-seats. It's amazing how more conservative the solo-pilot will be with another pair of eyes on them, qualified or not.
Lastly, was this incident caused by the F/O trying changing level using V/S? That only works on descent, in climb you stall, or get close. If you are new to the toaster (not-really-an-aeroplane-but-it-does-toast-pretty-well) use open modes for climb and descent. If in "coffin corner" select an airspeed between the zippers, give yourself a margin, and remember that one of those zippers will kill you -especially T-tails!!- the other will generate paperwork. As a lazy man I choose neither - be a lazy pilot and let the aircraft look after NOW so you can concentrate on NEXT.
Cheers,
Ned