Right, though he was not numbered with the apostles he was a disciple from the baptism of John and a witness of the resurrection.The other one likewise: "Joseph called Barsabas, who was surnamed Justus"
Yes, this is how I have perceived it, though I am also willing to consider that he was including Matthias, who was one of the twelve when Paul was writing 1 Corinthians 15.As a figure of speech: totum pro parte.