Your theory of Dumbledore's collusion with Snape in his death more or less coincides with mine. The idea that Albus was dying anyway is intereting and plausible. The guy was what--about 150 years old?
And your idea of the way the ghost-portraits work is interesting too, though a little more substantial than the way I think of them.
I'm glad you have Albus confide in Minerva. She deserved a boost after HBP.