I'm afraid that you've wasted your time with the Nanking cherries, according to this pollination chart. Besides, my Nanking cherries are the earliest bloomers of all my fruits (usually the first week of May, although it was mid-May this year), and they've finished blooming by the time the plum blossoms come out. I've got the same problem as you with a lone Pipestone plum, which is about to start its bloom. Fortunately, I've got pin and chokecherries in the middle of their bloom next door, and that should help. Do you have any in your area?

So I was able to buy some American plum scion’s (thanks Konrad!!) which I hope to graft onto that Pembina plum. Hopefully that’ll do the trick and I’ll be enjoying plums from that tree in the future!

I had the same problem until I grafted Sprout Sunshine on to the Pembina. I have 3 different plum grafts on this tree. One Spring I checked when the different grafts bloomed. Both the pembina and Sprout sunshine bloomed at about the same time. My Pembina is polinated by Sprout Subshine and has been for the last 20 years. I hope this is helpful.