Oh, yes, I definitely agree that Snape was there and cast the Patronus that night.

Actually I interpreted Ron's comment, at least from the movie, as that he'd been there for a day or two and was hoping one of them would show themselves. A Harry Potter timeline has Ron there the day before and so he's been in the woods all that time.

I think the sword had been in Snape's keeping but the sword then decided to move itself. Why it put itself at the bottom of a pond, I don't know. ;-)