First of all, dispite what the english version of AoS says, Soma is Japanese. I don't know why he is called an exchange student. Especially, because he and Mina are "childhood friends". I heard that in the Japanese version of AoS, there is no mention of him being an exchange student. In that version he is just a Japanese high school student.
Anyway, back on topic, the ending in Julius mode in DoS is just an alternate ending. From what I know, the real ending is the good one in which Soma destroys Menace (spelling?). What's truly depressing is that if there is another sorrow game made, I predict that either Soma or Alucard will die. I've always interpreted the prophecy from AoS in a way that either Soma or Alucard are the ones referred to. Think about it. The prophecy never said that the knew master would take power immediately. It only said that a new master would come to the castle and both Soma and Alucard were there. Also, those two are the only ones who have a legitimate right to the throne. Soma is Dracula's reincarnation and Alucard is Dracula's son. My prediction is that for one reason or another those two will end up fighting each other and only one will survive. The surviver will probably end up losing his humanity and become the new dark lord. Then again, this is only a theory.