'Useful' is about the highest compliment Sebastian can give. Well, that and 'tasty'.
"If I have given the impression of one who does not accept help, I apologise." He doesn't sound sorry; more gently matter-of-fact. Sure, Sebastian is used to having to do everything himself, but if someone proves themselves capable? He'll gladly take it.
"In fact, since you did offer earlier... In the event that I should fall asleep, I would be glad to be used for testing how effective the technique you mentioned is."
He could ask if it will hurt or if there are risks involved, but nah. Sebastian can handle anything. He'll need to make sure it's safe to use on Ciel, should it ever be needed.