In SI you could beat Jyggalag because you WERE Sheogorath, essentially. In otherwords, it was YOUR realm. Daedric princes are, for all intents and purposes, unbeatable in their realms (the player Sheogorath being a minor exception, since you were still mortal, as Jyggalag pointed out; however you could still stand against Jyggalag by virtue of being the Prince). However, they do have bodies (unlike the Aedra). That was Dagon at the end of Oblivion, in his true form. The Avatar was Martin. However, even though he was destroyed in Nirn, Dagon still exists; his defeat merely shut him back in his realm of Oblivion. So in short, no we will not be fighting Hermaeus Mora, especially not in Apocrypha since he would be essentially invincible.
As for this Dragonborn, the gift of dovah sos, Dragon Blood, can only come from Akatosh; that much is clear. However, WHEN this Dragon Priest received it is uncertain. Perhaps he betrayed the dragon priests and became Dragonborn/learned he was dragonborn (the mechanism is still not understood) by Akatosh's blessing but then became prideful in his own power and sought forbidden knowledge concerning words of power or the ability to preserve and eventually revive his being after death. Perhaps he was a Dragonborn who betrayed Akatosh and became a dragon priest but was lifted up in pride to make the dragons serve him and then killed them to consume their powers. Maybe we have this guy completely wrong. We'll find out soon enough