Certainly, Paul had a burden for "the lost". He would have probably been wearing you out with his preaching were he still here with us. ;o)
Paul did what he did because of love for the Lord and his fellow man. He did not want to see anyone condemned and even said he wished he could go to hell if it meant someone else would be saved from it. He disciplined himself so that no one could say he was a hypocrite and disregard his message. No, sorry, he was not in it for himself by any means. If that were true, he would have continued in his wealthy, privileged and snooty "religious" life because he was certainly doing just fine before his encounter with Jesus.
Finally, if you deny early church history about the fates of all the apostles and thousands of disciples because there is little extra-biblical documentation about them, then you must also deny everything else about the Christian faith. Oh...wait...you do. Doesn't matter, God demands faith to relate to him and some find that too much to ask.
I will not read his motives; he tells us pretty much what they are, and one thing that can never be denied about Paul is that he is pretty open and honest about things, so his intentions were not hidden it seems.
But he made it very clear that he will say, do and become anything in order to gain converts. Some can see it as a virtue, others as a vice.
Finally, if you deny early church history about the fates of all the apostles and thousands of disciples because there is little extra-biblical documentation about them, then you must also deny everything else about the Christian faith
You don't understand: not "little," there is nothing extra biblical about any of them. I don;t deny everything about the Christians faith. Obviously there are some early references to Christians and their practices form non-Chritsian sources, indirectly more than directly.
But as far as the apostles are concerned there is zilch. If you can find factual extra-biblical, extra-Christian mention of any of them I would be much obliged if you shared them with all.