The first two questions are easily answered.  The third might need some thought.  The reason it says what is says about Peter and the twelve is because he appeared first to Peter and later he appeared to all the apostles except for Thomas (John 20:19-23) and one week later to all the apostles, including Thomas (John 20:26-27).  If I said that my friend visited me last year and my family the year before, it would imply that I was included in “my family” the year before.  It would not make sense to say Peter then the apostles, including Peter.  That would be redundant.  The reason Paul calles the apostles “the twelve” is that this was a traditional designation of the apostles.  In Acts 1:23-26 a twelfth apostle to replace Judas was choses.  It is likely that Matthias was there when Jesus appeared to the eleven surviving apostles.  But even if he was not, “the twelve” was a common phrase to mean “the apostles.”

About the phrase “all the apostles” this probably refers to another later visit to the apostles.  Perhaps it is the one recorded in Acts 1:4-9.  The Bible occasionally uses the world apostle in a broader sense.  The word apostle means messenger.  In the biblical context it means a witness and messenger of Jesus.  Barnabas is called an apostle, as is Paul.  So, the phrase “all the apostles” may refer to a broader group of the inner circle of disciples of Jesus, not just the twelve.  Acts 1:4-9 is a good candidate for the resurrection appearance Paul is referring to.  If you read Acts 1:3 you can see that Jesus appeared to the believers a number of times, so it is hard to be absolutely sure which appearance Paul is referring to.

John Oakes