Plums and Pear are very low in nectar and that is the main reason bees stay away from. The best cross pollination I get when several..about 5 varieties are on one tree. I would think, cross pollination can get done by the wind.

Maybe when the blossoms are that close, wind may play a part in pollination. These folks from Australia discounted the notion of plum wind pollination and emphasize bees:
http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0019/117109/bee-cherry-plum-pollination.pdf

I do have 3 or 4 varieties grafted on a number of my trees and so have addressed the problem from that standpoint. My feeling is that when plums blossom here the temperatures are too low for adequate honeybee activity, and the insects are coming from hives not close to the trees while the mason bees might be active at lower temperatures and in any case would be quartered close to the trees. Plum blossoms might not be very attractive to bees but at the time of blossom here there's not much alternative food sources that I've seen. The Australians mention clover, but that's not flowering here at that time, and black cherry is later also.

Yes...same here, not much other food source except this, I rarely ever see a honey bee on the plum, the odd bumble yes and some kind of other native insect's like flies or ants I have seen crawling around.
Perhaps try to spray the tomato blossom set, some claim it works great, many years ago I have used it, maybe twice, I know that one's I had allot of fruit, ... didn't use it anymore
because I had more trees. I might try it again.