How about parting blow? If I have more than one in my hand, may I play them both as a response to one of my characters leaving play?

What if I had parting blow and blood for blood? Could both trigger from the same death?

Yes. A single trigger (such as a character being killed) can have more than one Response played for it. As many as you want. You just cannot use the same card to Respond twice (i.e., Robb Stark cannot just keep killing characters).