Jews accept the historicity of the events of the Hebrew scriptures only in part because of their being "eyewitness" accounts. The ultimate reason to believe the events to be historical is because the scriptures themselves are believed to be divinely inspired.
Similarly, Christians believe the events in the gospels to be true ultimately because they believe them to be divinely inspired. Divine inspiration is the "insurance policy", if you will, which guarantees their accuracy.
Regardless of what one thinks of the inspiration of the Jewish or Christian scriptures, the reports about the martyrdom of the apostles are not protected by divine inspiration. Consequently the are subject to a level of historical analysis which a believer would not apply to scripture itself.
It is clear Jesus was a real person
As it happens, I agree on this point.
it is also clear that the Apostles believed He was the Messiah
I also agree. Although I'd suggest that the apostles' understanding of Jesus was rather different than what was cemented as official church doctrine in the councils of the 4th century.