What I'm interested in is why clothing or lack thereof is bound up with the enslavement of the house elves: "This, sir?" said Dobby, plucking at the pillowcase. "'Tis a mark of the house-elf's enslavement, sir."

Why clothes? That's a funny spell. Does it imply that house-elves did not wear clothes before they were enslaved? Or that something about the clothes they wore was significant and had to be contolled? It seems more sinister than a simple uniform.

Re: the grapes. Harry was in the infirmary. It's usual to bring grapes to someone in hospital. I don't know why. It used to be traditional to bring a bottle of Lucozade as well (this probably doesn't pass cultural boundaries) until it was pointed out that bringing a bottle of sugary fizz to someone who was ill might not be a good idea.