How do traditional Christians defend the teaching that Paul is an authentic apostle? I want to preface this by saying I don't believe Paul was a false Apostle.  I must either view his Apostleship as authentic, or throw away everything I believe.  Removing Paul's writings and influence from the New Testament would change the message completely.

The reason I ask is that every so often, I have doubts about whether Paul was authentic, or an insidious infiltrator.  

When I have these doubts, they arise from several points.  


Saul (his name before his conversion for those few that don't know) hated Christianity.  He had been doing his best (or worst) to eliminate it.  He wasn't having much success with direct opposition.  Maybe he thought that if he got to be an insider with influence, he could corrupt the teachings from the inside?  (He wouldn't be the first or the last.)
Next, nobody witnessed his conversion.  He was on the road alone, and we have only his word for it.  Nobody can verify his claims.
Third, his teachings on salvation apart from works seems to contradict other clear teachings.


James 1:22-25
John 15:10
James 2:24
Matthew 5:19



Before posting this question, I did a quick Google search, and I did run across some who hold this position and make arguments for it.  Here's one.  The arguments are, at the very least, well-reasoned.

Again, I don't hold the position that he is a false Apostle, but if he is, then it has serious implications for my own beliefs, and for the validity of mainstream Christianity.  I understand just how "heretical" and "blasphemous" it must seem to even ask such a question, and just how deep the implications of what I'm asking go.  I'm hoping to find answers from others who have studied this out, or who have at least heard the arguments for and against such a view.
In Acts 9 there were witnesses when Jesus Christ spoke to Saul.


  3 As he neared Damascus on his journey, suddenly a light from heaven
  flashed around him. 4 He fell to the ground and heard a voice say to
  him, “Saul, Saul, why do you persecute me?”  5 “Who are you, Lord?”
  Saul asked.
  
  “I am Jesus, whom you are persecuting,” he replied. 6 “Now get up
  and go into the city, and you will be told what you must do.”
  
  7 The men traveling with Saul stood there speechless; they heard the
  sound but did not see anyone. 8 Saul got up from the ground, but when
  he opened his eyes he could see nothing. So they led him by the hand
  into Damascus. 9 For three days he was blind, and did not eat or drink
  anything.


Also, we have from another disciple, Ananias, whom God told Saul was His chosen instrument to proclaim His name to the Gentiles.


  15 But the Lord said to Ananias, “Go! This man is my chosen instrument
  to proclaim my name to the Gentiles and their kings and to the people
  of Israel. 16 I will show him how much he must suffer for my name.”


Paul did not act completely on his own, either.  He had the full support of the apostles.


  Acts 15:22-27 NIV
  22 Then the apostles and elders, with the whole church, decided to
  choose some of their own men and send them to Antioch with Paul and
  Barnabas. They chose Judas (called Barsabbas) and Silas, men who were
  leaders among the believers. 23 With them they sent the following
  letter:
  
  The apostles and elders, your brothers,
  
  To the Gentile believers in Antioch, Syria and Cilicia:
  
  Greetings.
  
  24 We have heard that some went out from us without our authorization
  and disturbed you, troubling your minds by what they said. 25 So we
  all agreed to choose some men and send them to you with our dear
  friends Barnabas and Paul— 26 men who have risked their lives for the
  name of our Lord Jesus Christ. 27 Therefore we are sending Judas and
  Silas to confirm by word of mouth what we are writing.


Since I trust the original apostles were acting on Christs behalf, I also trust that Paul was.  As you said, if Paul was a false apostle, then it would hold serious implications for our beliefs.  It would seem that if he was false and also supported by the apostles, the truth of the apostles would also be called into question.