Sorry, MMM - no definite recipe either. You are right, everybody seems to do it differently. I hadn't heard about taking the kernels out for plum jam, so I don't think it has anything to do with pectin, more with flavour...
When I was in France, we made a lot of greengage jam. Traditionally, they made it with ordinary sugar, and boiled for quite some time. I bought jam sugar (the stuff with pectin in), used it in the ratio 1:1 with the greengages, and boiled for only 5 minutes. Both types of jam were very nice - their's was mushier, mine more bitty and with more bite.
In Germany, we had more of the blue type plums. I either used the jam sugar (as above), for which you need to cut up the fruit in bits or stick a liquidiser in for a while. Or I made the traditional "Pflaumenmus", for which you need next to no sugar (it works with none at all, but we often used a ratio of 1:10 sugar:plums), and which is boiled on a very low heat (or in the oven) for anything up to 4 hours. A lot of liquid evaporises in that time, so you end up with very little jam in relation to the fruit you started out with - but a very concentrated flavour. I like to add a bit of cinnamon and cloves to both types. My aunt swore by adding a green walnut.
Sorry, this has probably left you with even more options...
). Strangely the birds aren't eating them this year?