When Doran and co. learn how Quentyn died it will be only a matter of time before they join "Aegon", I think. And assuming he buys the story about "Aegon"'s parentage, he might feel that this is the only reasonable course of action. Dorne never cared about Dany's claim, they wanted to join her simply to take revenge on the Lannisters for the deaths of Elia and the kids; with one of Elia's kids around, Dany becomes useless for them.