Probably not. There must be a good reason for laws prohibiting commercial selling and slaughter of horses for human consumption. Plus it's a bit weird, considering that people ride them and make friends with them, etc. Kinda like pets? We don't eat pets, do we?
Yes. No reason to waste a good kidney. I certainly wouldn't accept the brain if that were ever a thing, because that's what's basically being punished and wasted.
Would you accept your friend's face as a sure, guaranteed transplant with no glitches, if both of you were in an accident, your friend didn't make it and you lost your face?
I wouldn't reply to a question I don't want to answer. There's never a need to lie and there's no requirement to participate. Just don't reply to questions you don't want to answer. ¯\_(ツ)_/¯