The apostles were the apostles, not necessarily because they were "hand-picked by Messiah," but because they were first-hand eyewitnesses of Messiah's life from the very beginning of His ministry:
and ye also bear witness, because ye have been with me from the beginning. - Jn 15:27
Ye are witnesses of these things. - Lk 24:48
But ye shall receive power, when the Holy Spirit is come upon you: and ye shall be my witnesses both in Jerusalem, and in all Judaea and Samaria, and unto the uttermost part of the earth. - Acts 1:8
This was confirmed by the Apostles John & Peter:
And the Word became flesh, and dwelt among us (and we beheld his glory, glory as of the only begotten from the Father), full of grace and truth. Jn 1:14
That which was from the beginning, that which we have heard, that which we have seen with our eyes, that which we beheld, and our hands handled, concerning the Word of life (and the life was manifested, and we have seen, and bear witness, and declare unto you the life, the eternal [life], which was with the Father, and was manifested unto us); that which we have seen and heard declare we unto you also, that ye also may have fellowship with us: yea, and our fellowship is with the Father, and with his Son Jesus Christ: ... And this is the message which we have heard from him and announce unto you, that God is light, and in him is no darkness at all. - 1John 1
And we have beheld and bear witness that the Father hath sent the Son [to be] the Saviour of the world. - 1John 4:14
The elders among you I exhort, who am a fellow-elder, and a witness of the sufferings of Christ, who am also a partaker of the glory that shall be revealed: - 1Pet 5:1
For we did not follow cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. - 2Pet 1:16
for we cannot but speak the things which we saw and heard. - Acts 4:20
Matthias was chosen to replace Judas because he was one of two eyewitnesses from the very beginning of Messiah's ministry, as was required for the original apostles:
Of the men therefore that have companied with us all the time that the Lord Jesus went in and went out among us, beginning from the baptism of John, unto the day that he was received up from us, of these must one become a witness with us of his resurrection. - Acts 1:21,22
Paul was not an eyewitness to Messiah's ministry.
Stephen could not have been an apostle. There is no evidence that he was a witness, nor did he know that Torah was ordained by YHWH - not by the angels, as he claimed! (Acts 7:53)
I doubt it. Paul did not betray the Pharisees. In fact, he confessed that he remained a Pharisee well into his "Christian ministry" (Acts 23:6, Phl 3:5).