The conceptual scheme that allows this possibility was set out in the Court's judgment, delivered by Charron J, at [33-42]. The first point is unexceptional: there are two categories of abuse of process – those which concern trial fairness, and those which raise the integrity of judicial process . The unsettling thing comes next :
The placing of trial fairness (that is, not public policy fairness) in the first category of abuse of process and within the balancing exercise was purportedly illustrated at [39-40] by reference to a case concerning whether requiring the defendant to undergo a third trial after two juries had failed to agree was an abuse of process. That case, R v Keyowski, 1988 CanLII 74 (SCC),  1 S.C.R. 657, was really about public policy fairness. There was no suggestion that a third trial there would not itself be a fair trial.
It is easy to confuse trial fairness with public policy fairness. I suggested that the Privy Council did this in Boolell v The State (Mauritius)  UKPC 46, noted here on 18 October 2006.
Canada recognises that the defendant's right to a fair trial is an absolute right, not subject to balancing: for example R v Ahmad, 2011 SCC 6, discussed here on 22 February 2011. This makes the framework of analysis set out in Nixon rather misleading.
Aside from that difficulty, Nixon makes some useful points about the scope of prosecutorial discretion and its relationship with the court's duty to prevent an abuse of process.
I should add that while a balancing of competing interests will determine whether public policy supports a finding of abuse of process, a balancing of competing interests will not determine whether a trial was or would be fair. Yes, balancing of subsidiary rights may be required in order to resolve an issue, but once that balancing is done the result must be assessed for compliance with the defendant's right to a fair trial. An illustration is R v H  UKHL 3, at , concerning whether a trial could be fair without disclosure of the identity of a witness.
Furthermore, abuse of process as originally conceived was thought to cover a relatively narrow field of wrongs, but it has since been recognised that it is a more general concept. Once one adds within its scope the issue of trial fairness, this extended application is evident. Any substantial miscarriage of justice can be said to have caused trial unfairness and so be an abuse of process. That includes anything that would give rise to a successful appeal against conviction. More interesting is the choice of remedy once such an error has been identified. Can it be put right by a warning to the jury? Should it result in exclusion of evidence that otherwise would have been admissible? Or, most drastically, should the proceedings be stayed? Different forms of decision process apply to the choices. When a judge has to decide whether to warn a jury, or to exclude evidence, the decision is reached by the well known weighing of probative value against risk of illegitimate prejudice. When the choice is between warning the jury and staying the proceedings, the decision turns on whether there would be an unacceptable risk of an unfair trial, and that is not a balancing exercise. When the choice is between excluding tainted evidence and staying the proceedings, the decision is one of public policy balancing. I have discussed this in more detail in "The Duty to Prevent an Abuse of Process by Staying Criminal Proceedings".