Currently, the damage increase for the Pestilence node of Spirit Plague stacks multiple times for each recast or spread. The text on the node does not say that this damage bonus should stack unlike other nodes like Horrific Fascination.
Also, when you proc Hungering Souls with Death Seal, the crit from Hungering souls does not trigger the Darkguard passive. You don`t get any ward at all. I:m not sure if this is an intended interaction. On the other hand, Rip blood crits when triggered by the Transplant node can generate ward from Darkguard.