In the detection test of nitrogen,the Prussian blue colour is due to which compound?

In Lassaigne's test for the detection of halogens,the sodium fusion extract is first boiled with concentrated nitric acid. This is

Given below are two statements :
Statement $I$ : In Lassaigne's Test,when both nitrogen and sulphur are present in an organic compound,sodium thiocyanate is formed.
Statement $II$ : If both nitrogen and sulphur are present in an organic compound,then the excess of sodium used in sodium fusion will decompose the sodium thiocyanate formed to give $NaCN$ and $Na_{2}S$.
In the light of the above statements,choose the most appropriate answer from the options given below.

Why is nitric acid $(HNO_3)$ added to sodium extract before adding silver nitrate $(AgNO_3)$ for testing halogens?

What happens to the halogen when $AgNO_3$ is added to the Lassaigne solution acidified with $HNO_3$?

Which of the following compounds is not expected to show Lassaigne's test for nitrogen?