Reely,
The TFO's, legally, are passengers. To me, as a police pilot, they are definitely crew - if they are not in the aircraft, there is not much point me going anywhere.
The "Tasking" comes from the police control rooms/NPAS dispatch. Any task given has to be achievable and a good use of the resource. As a pilot, I can only refuse a task based on the serviceability of the aircraft, the weather limitations and the abilities/capabilities/legalities of the aircraft and it's crew. Whether or not I think its a pointless exercise or waste of time and/or resources, it is up to the TFO's to use their knowledge and experience to sort the wheat from the chaff.
Achievable also means, can we get there, devote a reasonable time to the task and then get back to fuel? I tend to fly around with Base as an active waypoint on one of the GPS's so I always have an indication of distance to travel and then use 3 kgs per min to plan on.
As for your question about the "company policy", ask the company.