The necropsy results from the September 3 Mallard deaths on Tobin Court in Peterborough were released by MNRF on December 16, 2017. They ruled out poisoning. They did extensive testing for botulism, which is a reason that the results took so long. The results for botulism were negative, as well. However, they did indicate that botulism is extremely tough to test for and can result in false negative test results. So, it looks like we will never know what exactly happened but, if anything, the neighbours can be assured that it was not as a result of poisoning.
N.B. More information about avian botulism and cholera can be found HERE.