Of the 12 Apostles, and James (brother of Jesus) and Paul, we can be confident at the very least that James (brother of Jesus), Paul, Thomas, John Son of Zebedee, Peter, and James Son of Zebedee were martyred because we have early testimony. While the other Apostles may have been martyred for their eyewitness testimony, the dating of the material we have is too far removed to be sure.

They were proclaiming the risen Jesus from the start in Acts 4 & 5. Acts is a biography of Paul but makes no mention of his death, so we know Acts was written before 65 AD when Paul was put to death.

James Son of Zebedee was killed in Acts 12.2. Paul wrote he was told to stop preaching the risen Jesus.

Was Jesus seen by skeptics? Yes. Thomas, James (brother of Jesus) and Paul did not believe in Jesus until they saw Him alive from the dead. Of the 500 surely some of them did not believe.