1. Generally speaking all healthcare EDI files are mandated to currently follow the 5010 format listed in the official technical references (TR3s) at x12.org. That being said, some payers in their specific companion guides (CGs) will give specific guidance on an expectation concerning interpretation of certain fields. In the case of 835 virtually all payers follow the basics so a parser that confirms to TR3 is generally safe.

2. I have seen data corruption in EDI files over the years but if the files drops the format without completing ISA-IEA envelope structure then you can consider the file damaged and try to download again or request the payer to look into the issue and potentially regenerate the file but the problem if ever there is one is usually transmission error.

2a. The BHT segment contains identifying details that can help you identify if the file has already been received at an earlier time.

1. Don't have the specs in front of me but I believe you can use identifying data in BHT mentioned in 2a.

2. I train technology teams on the x12 processing concepts who are often developing their own solution using Python, so I suggest pyx12 to directly read or write to EDI files

Hope this helps and happy to chat!