Is the money earned from the monthly "shift" allowance halal, knowing that the conditions of the allowance (12 hours of work daily) do not fully apply to the nature of the additional work (two hours of pitching tents after official working hours) with which we are tasked?

If the person responsible for determining bonuses is authorized to do so, then there is no harm in benefiting from the rewards he gives. However, if he is not authorized and the system does not permit it, then it is not permissible to take what he gives that is not deserved.