OT Truth,
before I get to the questions you ducked, consider this way of linking Jesus' resurrection to eyewitness testimony:
Paul introduces the sequence or resurrection appearances with tradition transmission phrases traceable to the eyewitnesses:
"I handed on to you as of first importance what I in turn had received." Received from whom? From the Jerusalem apostles!
Paul is referring to his 2 trips to Jerusalem to have his Gospel checked out for accuracy by the eyewitnesses and says they vouched for the accuracy of his presentation (Galatians 1:18-19; 2:1-2, 6-10). So here is the list of resurrection appearances that the apostles confirmed:
"...that He was raised on the third day in accordance with the Scriptures, and that He appeared to Cephas (= Peter), then to the Twelve, Then He appeared to more than 500 brethren at one time, most of whom are still alive, though some of died. Then He appeared to James, then to all the apostles. Last of all,...He appeared to me (1 Corinthians 15:3-8)."
Luke's Gospel confirms Jesus' first appearance to Peter (Luke 24:34). All 3 Gospels with Easter appearances confirm that Jesus' 2nd appearance was to the 12 (a technical term for the apostles in general, excluding Judas Iscariot). Paul is personally acquainted with some of the 500+ who saw the risen Jesus, and so, he is aware of the recent deaths of some of these. Each Gospel is selective in the appearances they record.