Once you have used some of your fixed reserve you then fill in an ASIR to explain why you needed to!
Not necessarily. The requirement to make a report is dependent on whether the fuel state became "dangerously" low or not.
Most company ops manuals I have seen stick to the CAAP or something very similar. Private Ops I guess you can burn it but buyer beware, if something goes wrong you don't really have a leg to stand on, as the lawyers will reference the CAAP.
The lawyers can't get you for burning fixed reserve if you are a private operation. It does not matter what the CAAP says in this case.
For a commercial operation (that references the CAAP in its fuel policy), however, if the lawyers are after you, it won't be because you deviated from the fuel policy, it will be because you ran out of fuel.
In which case, the rule they will get you on is CAR 234, which states "thou shalt not run out of fuel".