As to the origins of the OT - I watched this on youtube:
https://www.youtube.com/watch?v=O5RfScpEcZ8
I'm not competent to comment on its accuracy, but it's compelling, and sits well with other work by (for example) Francesca Stavrakopoulou which casts doubt on the veracity of the stories of David, etc.
At about 1:20 it suggests that the Torah (the Pentateuch) was written and promulgated in 458 BC by Ezra. This would, perhaps, make the OT as reliable historically as the NT.
Worth a watch, I suggest.
(PS: In a comment which is entirely beneath me and irreverent, I recommend Francesca's you tube videos for her scholarship, communication skills, and mammaries.)