Before we start getting into a giant debate about the best way to interpret the evidence, let's lay out exactly what evidence we actually have.

What precisely does the JC Bible claim? Is there any physical evidence or independent documentary evidence?

Most of the evidence of martyrs does not actually come from the JC bible but rather from the accounts of the early Fathers of the Church. It is of varying reliability. Some testimony of persecution (and martyrdom), such as seven of the letters of Bishop St. Ignatius of Antioch are almost certainly genuine as well as the 'Apology' of Justin Martyr as well as an account of his death. Certain other accounts of martyrs are perhaps in a more disputed category (some of Eusebius accounts perhaps?) while some are much later fabrications. What I'm suggesting is that perhaps the JC NT is not the only place to look or even the best for accounts of martyrs.

Are these martyrs witnesses to the events of the Bible?

Remember the claim is that "N people witnessed the events of the Gospels, and their martyrdom is evidence of the sincerity of their testimony."

I want to see what evidence there is that claims that people did witness the events of the Gospel and were subsequently persecuted and killed rather than abjure their testimony.

James, the brother of John, was killed by Herod in 41 CE presumably for his beliefs. This is recorded by Eusebius, who also quotes Clement, and if I recall correctly, it may be mentioned in Josephus. It is also mentioned in Acts 12:1-3. So this incident is probably fairly likely.

It appears that James the brother of Jesus was also killed for his beliefs by being thrown off a wing of the temple and clubbed.His death is also mentioned by Eusebius who cites Clement again and quotes from Hegesippus.

Peter is reputed to have suffered martyrdom at Rome under Nero. This time Eusebius quotes Caius. It is thought that a redactor of John is referring to Peter's death in 21:18-19.
Although many other apostles are traditionally thought to have been killed for their faith, these are probably some of the more reliable accounts.

Opus1

layman wrote:
Fine. Please prove this to be true. Show me evidence that Stephen, Peter, and James died because they stated the physical resurrection of Jesus to be an historical fact, and that by recanting of this belief they could have escaped persecution.

My opinion, and that of many scholars, is that most early Christians (including Paul) believed in a spiritual resurrection. That is, they "saw" Jesus much in the same way that modern believers "hear" God. They believed that Jesus would return very soon to usher in the kingdom of Heaven. His appearances to them were to give them signs of how to live in the meantime, and the details of his coming again. The idea of a physical resurrection did not yet exist.

But then the gnostics sprung up. They also believed in a spiritual resurrection of Jesus. But their Jesus did not just appear to James, Peter, and the original disciples but kept appearing to people who had never met him. There is substantial evidence that gnostics based beliefs and behavior on their own visions of Jesus, or the visions of their leaders.

As such, by the time we get to the later gospels (Luke and John), it has become necessary to distinguish between the appearances of Jesus to his close disciples, and such later appearances. So the orthodox authorities devised a distinction: Jesus had bodily appeared at first, and then was bodily ascended to Heaven after 40 days. All subsequent appearances were non-authoritative, and should be disregarded in shaping personal behavior and church policy, unlike the earlier bodily ones.

Thus, when James, Peter, and the other early Christians were martyred, the idea of a physical resurrection was a non-issue. It did not even exist.