Re: Double negative
I can't speak to French usage, but perhaps the confusion was due to "corn" nearly always meaning maize in US English, but in British English (and historically, prior to the adoption of maize as a staple in the US) it was broadly used for a variety of grains, and even for non-grains such as salt pellets (hence "corned beef").
So perhaps a French speaker translated blé to the British English "corn" (non-specific but not incorrect), when "wheat" (specific) was wanted, and then in communicating with the US this was interpreted as maize?
Still, you'd think this would have been sorted out before shipments arrived. After all, there are a number of varieties of maize - feed, sweet (for human consumption), pop, etc - and preparations (on the cob, dried kernels, canned, masa, hominy, and so on). In working out the details it seems like the mistake would be uncovered.