I hadn't considered Slughorn. I don't know, though...he knew DD was after information about the horcruxes--after all, he didn't want to part with the memory that cast him in a bad light for having shared information with Tom Riddle. Wouldn't he have told DD if he'd helped destroy one? Someone HAD to have been with Reulus. I don't see how he could have managed it on his own.
What about Lily? She was good at potions, after all.
I've just reread your post, now I realize that is exactly what you were saying, regarding Lily, not Sluhorn. Sheesh! I'm tired!!!
I'm still wondering about the sword being the only relic of Gryffindor. The sorting hat was Gryffindor's as well, I think. In one of the books, it sings about Gryffindor taking the hat off his head and the founders making it into the Sorting Hat. Does it not count as a relic? And the sword was delivered in the hat for Harry to use in battling the basilisk and the diary horcrux. Odd, isn't it? I always wondered why the sword didn't just materialize, but had to arrive via the hat.
I tend to agree that Dumbledore was telling Snape to kill him. Remember too, that at one point Hagrid lets slip that Snape and Dumbledore argued about something Snape didn't want to do anymore, and Dumbledore telling Snape he had to do it... did Dumbledore know what was coming? Seems likely to me that he somehow had made Snape swear to kill him if need be.