Ok, the plumber came over today. He took apart the Grundfos pump for the 2nd floor zone that I've been having overheating problems with. He said that it looked like there was some hard water build up inside and that he cleaned it. I doubted this would solve the issue, but he's the expert, and I'm just the home owner. I asked if that if the forward and return manifolds should be connected and he advised me that they were already connected inside the LP boiler. He politely said that the whole issue could be operator error. I've been reading everything on this thread and others too, especially the posts from mikefrommaine, Clarkbug, and flyingcow, and felt pretty good when I defended the "operator error" claim. The plumber provided me with the schematics he used.