Ben, whether any incident results in a Mayday or a Pan call, whether it calls for an immediate landing or not, are questions for the judgement of the Captain of the aircraft concerned. There are very few hard and fast rules. Any Captain's judgement may be called into question later, but, as we all know, hindsight is 20-20 vision. He is the man on the spot, and that's why he's paid what he is - to exercise judgement in the heat of the moment.
If you remain confused about the distinction between Pan and Mayday, perhaps it will help to look at them as the instant severity of the problem that a pilot faces. It is not relevant that a currently severe problem may later be cured and the flight safely continued.
I can imagine many scenarios in which I would want an immediate landing after making a Pan call, and continue after a Mayday.
Hope this helps.