The Dumbledore-Snape Plot Revisited

All right. Reading so much fan fic over the past year has made me a subscriber to the theory that of course Snape killing Dumbledore was part of a plot that Dumbledore agreed to, or hatched, or whatever.

But this morning I woke up doubting myself on that score again. First of all, if Dumbledore knew he was going to die, how is it that he's offering Draco and his family protection? Had Draco fallen to his knees and been like "you're right, I'm wrong, save me now" would the outcome have been different? No. Snape still would have finished him off and then fled with the young Malfoy.

Second, the most likely plan we keep hearing is that Snape needs to be in place close to Voldemort so that when Harry arrives to finish him off, he'll be able to help. But didn't Dumbledore tell Harry to tell no one about the horcruxes but Ron and Hermione? Presumably it's going to take an entire year (an entire book), to find them all and destroy them. Snape's going to be wondering what is bloody well taking so long...?

All right, start poking holes in these theories. I'm just having one of those, what if Dumbledore really screwed up? moments.