Quickbooks support used to be great. As anyone reading this thread can see... they now.. are not. 

After 4 months of headache and well over 40hrs invested, I stumbled upon the simple (maddeningly simple) solution:

- IF YOUR CSV FILE ENDS IN ALL CAPS, LOWER THEM TO LOWERCASE. (i.e. "paypal-export.CSV" will not work; "paypal-export.csv" will)

Why not a single support agent or QA tester could locate this solution is beyond me.