With the prophecy thing, I think taht he just wouldn't trust his DE's that much... and remember, he wasnt' supposed to be at the MOM... i think that he showed up because he didnt' trust them to pull the job off properly... (which they didnt'
)
And about the thestrals, I didnt' understand why he coudlnt' see them before either. Okay, so maybe he didnt' comprehand the death, or remember it when he was one, but after 3rd yr, with the memories bought about by the dementors, he definately should have seen them then, no???