The plum definitely looks like it has died. I'm sorry about that. The second one also looks stressed due to the early color change of the leaves - the foliage should be dark purple. You may also lose that one. In looking up growing conditions for this plant, drainage was a big factor - it won't tolerate poor drainage, something that occurs in clay soil. So your plants may be struggling in their current site. You may want to dig up the dead plant and take a look at the roots. I would hazard a guess they are circling around and have not grown outward as trees should. Feel free to send more pictures if you choose to do this.

As for the second tree, it may be too late to save it. You would have to loosen the soil carefully around the root area as deeply as possible with a garden fork. I am not sure it would even help. 

If you remove both these trees, I would not replace them. The white pines you have there appear to be growing pretty well. Unless these pines are dwarf conifers, a mature white pine will grow to 50-80 ft tall and 20-40 ft wide and will overgrow anything planted nearby. You could plant some tall perennials between the pines to fill in while the trees grow, but I wouldn't recommend planting more trees or shrubs there.