I suspect you have a particular view of what Paul as an "imposter" means.
In any case, you clearly have a very relaxed view of what constitutes an "independent source" if you think "Luke" the Acts author, who was supposedly a companion of Paul and most likely actually a pupil of Paul, could give an independent account. Where do you think he sourced his accounts about Paul ?
Peter was already convinced that the wider ministry must include Gentiles despite the dominant opposition at the time of those who claimed Jesus belonged only to the "circumcised".
To find an advocate in Paul must have been a great relief.
