There should also be a paper trail of the tech logs and fuel receipts
I would not put too much hopes on paper trail of Fuel receipts. knowing the inflation rate and actual use of local currencies in that part of the world , fuel bills are most likely to be in US $ . Now seen the financial status of the airline I doubt they did have a fuel credit, and if they did not, they most probably have had to pay their fuel bills in cash on the spot.
Not been to Bolivia or Venezuela recently , but when I was last, when handling US $ you got a handwritten paper receipt not worth much in modern accounting, and negotiations started on how much you wanted written on it.
And the economic situation was much better then than it is now.