I dont have that deep knowledge of the subject, only stories from historians. Problem here in Serbia is that you never know which historian is real one, and which one tales you nationalistic version, so I cant claim that stories from my teachers are true. In the books that I stuied from it is said that Dusan death isn't certain but that it is possible that he was poisened.Was this claim of Dusan acknowledged in the West? I suppose it wasn't in Constantinople.
Now one historian said at class that Pope and Dusan negotiated about launching a crusade, that Dusan would lead ( that would probably mean he was or he was going to be acknowledged by the pope). Otherone aded that pope required from Dusan and his court to convert to Catholic in order to lead a crusade which he refused. And there is even a theory that Pope ordered to poison Dusan becouse of his refusal to convert. I cant verify nothing of this, stricly becouse Im sceptical about history unless I get a confirmation from multiple sources. I hope one day I will get the time to research deeper into subject, from an Europian sources, but have no idea where to begin.