While I know that ECC corrects memory errors, does the non-ECC memory of the GPU ultimately rule on export/render?
You're right, it does. A memory error may occur on the GPU memory as well as on the ECC system memory. While single bit error will be corrected in the system memory, on the GPU memory you won't even be notified of the error. The faulty bit is simply copied back from GPU memory to ECC system memory.