I've spent years automating exactly the type of work described.
I sit next to a person who does their job, keying all day, from a variety of sources.
They have their own mental business rules that have to be documented. All those cut&pastes of rows of spreadsheet columns, and the occasional cell overwrite killing you happened all the time.
They hated me, know I will be automating the task they are doing, so they always exaggerated the difficulty, hoping I will give up.
No dice.
So they do it very quickly, skipping over and redoing for mistakes that I couldn't catch, then abuse me for asking them to explain what just happened. I was just too slow or stupid to handle it, and obviously when they complained to their boss I'd stop distracting them from their job.
I have sat through this performance with DOZENS of people through my career.
Most of the time I give up on THEM. I document what they do from and input/output blackbox perspective, and replace it all from scratch.
I ALWAYS found errors in current implementation of business rules, either in that they produced obvious erroneous results, or they didn't follow the spec. If I was to tell the customer (always working on a customer system), then we'd have to refund a shitload of money. So we didn't.
But if I changed the system to be "correct" in my version of it, then the next month's production run would vary too much from the previous, setup red flags, and then be investigated by the customer.
So replacement systems would be bug-for-bug compliant until I could slide the fixes in without being noticed.