What usually happens is that the pin seizes in the door bush and so when you open the door the pin moves at the same time and the sliding surfaces are between the aluminium bushes in the body (lower sill, upper door aperture). This is what causes the wear and doors to drop.
My first shot would be to sit inside the car, hold the pin with mole grips and try to open the door. If it moves the mole grips as well, then use the leverage of the door whilst holding the grips to get the pin rotating while the door is stationary. That'll be the best way to get the release fluid where it's needed and any marks from the grips can be filed off before you start to drop the pin out.
Brian
Been out all day, but popped out to the garage this evening to see if I could make any progress. With the driver's door supported, got the split pin out!
However, the door pin rotates with the door, and so far, is resisting any attempts with the mole grips to loosen. Will try again tomorrow. The passenger door pin, on the other hand, does not rotate as you open the door, but the crusty tail of the split pin is pointing inwards, so removing that is going to be joyful, unless I can get an improvised punch on it and knock it through. One for tomorrow.