I'd like to get back to the phraseology issue.
Pan, pan, pan denotes an urgency: no immediate assistance is required.
Mayday, mayday, mayday denotes distress: immediate assistance is required.
What is the point of declaring a fuel pan (thinking you are landing with slightly more than the reserve fuel) if you do not need ATC assistance, ie priority? I would always go for mayday and associated paperwork.
Think you've answered this yourself. A fuel PAN says that the assistance is required when you reach the destination, be it a diversion airfield or not. If you're already en route to that airfield then you will receive a priority approach and landing, therefore
immediate assistance is not required. A Mayday call means "get me on the ground now by all means possible", and to be honest probably means something has gone badly wrong. I can think of numerous scenarios which could generate a PAN for fuel - more holding than expected due to blocked runway/snow clearing etc. or diversion as in this case (wholly appropriate, IMHO ), but not many which would generate a MAYDAY.
