Here’s how the rule of law is supposed to work.
The parliament passes a law that makes it a strict liability offence to wear a pink bunny suit. The evident policy of that law is that is an offence to wear a pink bunny suit, whether or not the wearer intended to wear the pink bunny suit. Doesn’t matter why you’re wearing the pink bunny suit. Whether you did it deliberately, or inadvertently donned your pink bunny suit rather than your jeans and T shirt or got drunk and ended up in a pink bunny suit because you couldn’t care less - doesn’t matter. (There are, of course, exceptions to strict liability offences. For example, you would not commit an offence if someone held a gun at your head and said they would kill you if you didn’t wear a pink bunny suit.)
Note that this outcome is not a consequence of the exercise of any discretion on the part of a regulator.
If sufficient people disagree with the policy of a law that makes it a strict liability offence to wear a pink bunny suit, the parliament will repeal the law or change it so that an element of the offence is the “why”.
In the case of the offences that will be created by the fuel Instrument in November 18 in Australia, if it comes into force in its current terms, it similarly doesn’t matter why you arrived at your planned destination with less than the FFR or failed to divert to an alternative to ensure you landed with FFR intact. The offence has been committed (unless someone was holding a gun at your head, in which case your actions were not voluntary).
You seem to think there’s another element of the offence - the “why” - whereby a benevolent regulator is going to exercise its discretion not to refer a matter for prosecution or use it to justify administrative torture, on the basis of the regulator’s judgment as to the merits of the “why”. That’s not consistent with the principles of the rule of law (and is also why CASA has the reputation that it does). If the “why” is relevant, it should be built in as an element of the offence, so that the pilot population knows when it will break the law and when it will not, irrespective of CASA’s opinion on the matter.
In your scenario there is a “suitable en-route alternate available”. In the scenario I gave there isn’t (and an alternate did not have to be planned). The PIC in my scenario nonetheless commits a strict liability offence for arriving at the destination with 29 minutes’ reserve, it being impossible to divert to an alternate and land with FFR intact when the estimated fuel state on arrival at the destination became clear.
The question whether the PIC in my scenario is referred for prosecution or administratively tortured for this heinous crime becomes a matter of discretion for the regulator. That’s a bad outcome.