Why would you plan to kill someone in a plane crash, but poison them first? sheesh. You can make the plot as elaborate as you want, but at some point it just becomes silly.
Since the plan was to have her fly to the other island, and that the plane's take off was delayed, maybe the plan was not to have the plane crash, but to arrive at its destination in no shape to take off again, losing power on final or the pre-take off run up.
Given that, poison administered before the flight would have had time to act, the 'distress' would have occurred on dry land, without means (or requiring alternate transport--all time consuming) to get back to better medical care in a timely fashion, leading to Fuddy's demise.
Enter Mr. Murphy (of Murphy's Law), and the take off is delayed, the malfunction occurs over water, which has the same net effect, and Fuddy checks out while in the drink, provided the accounts are even true (there are inconsistencies in the story).
Alternatively, the plane was intentionally disabled in the air, the ditching was intentional, arrhythmia was induced by the administration of either injected or inhaled drugs (the amyl/butyl nitrate) leading to Fuddy's death (which would require a minimum of one accomplice on board, and a very competent pilot who need not have been in on it).
All these are possibilities, not a theory.
What consistently bugs me is that arrhythmia is cited as the cause of death, in someone who had no heart problems. Not myocardial infarction, but arrhythmia. You gotta have a beat to not have rhythm. How could that (and by all means, someone more qualified in cardiology than I am, correct me if I am wrong) arrhythmia be seen in autopsy? Especially if not previously diagnosed?
Sudden cardiac failure? Okay. But arrhythmia?
Or alternatively, Fuddy just checked out, her number was up, hit her breath limit. The fact that she could have been a vital witness, co-conspirator, or otherwise shed light on the very questionable alleged 'fake but true' birth documents of the usurper in the White House possibly leading to the negation of not only his 'executive orders' but a reversal of a host of UnConstitutional strides toward totalitarianism in this country is just a coincidence.
...too many feathers there to not cry fowl!