Hello! I'm not a native speaker or teacher but I like to take part in such discussions.
It seems to me that you understand the difference(s) between Present Progressive and Present Simple.
In this sentence...Jack is playing the guitar at the concert that we are going to...I'd rather say that Jack will be playing the guitar...It sounds more naturally to me, though may be I'm no right and Present Progressive is quite acceptable here.
Present Simple is not good there either in my opinion.
Student or Learner