Gregorian chant, a more organized form of plain song, was introduced by Gregory (duh obvious answer). It went through various changes in Italy. St Ambrose is credited with introducing plain song (but records unclear); this form of unified chant may be older. As one word, plainchant is not very old.
Guitar- the cithara goes way back, probably with the lyre as antecedent, but at which point in its evolution you would call it a guitar, I dunno. I would say false.
I agree with Wolfgang on the sword dancing.
The first morality play. I performed in a medieval German morality play when I was studying German. This would be a tough one to track down.
I am afraid I would have to look up the rest that remain unanswered, and that would b cheating (only done with crosswords).