This is a very complex case, I would suggest to create a support case to have a Cypress engineer answer that. At top of this page "Design Support -> Create a Support Case".
To quote the TRM:
In both the cases, a DMA channel of low priority can be interrupted by a high priority channel only during the source engine phase
So a running DMA transfer cannot be interrupted. My suggestion: configure your DMA so that only single bytes / words are transferred in a single request /burst. That way after each transfer the ADC transfer can get priority.
Well, I know it's not really the solution to the question I had asked in the case someone ran into similar issues, but in my particular use case, the DMA to non-volatile was actually not needed after I had done some optimization to my code. I was able to clean it up to the point were the dma handled the ADC samples and the non-vol write was done by the processor and still meet real time requirements. I would like to revisit this given time.