It would seem to me that DD knew and had possibly planned with Snape that Snape would do him in. That might explain why DD was telling Harry to go get Snape when they returned to Hogwarts. There isn't too much that DD doesn't see. He seems to always know when Harry has something on his mind doesn't he? DD is kind of like Obi-wan in a way. He is always going to be around now to help Harry with whatever Harry has to do just in picture form like the other past headmasters. I also think that Harry and Ginny will stay together and that will be the love that Harry has that LV doesn't and that will be how Harry finally defeats the dark lord.