My understanding is the Choshen (Breastplate of Judgement) was to be used by either the King or the Cohen Gadol and would decide issues of either 1) national importance or would be used to 2) correct a judgement made by the courts. My question is why did it the certain letters illuminate for Eli the Cohen when it seemed to not be for either reason, national importance or correcting a court judgement regarding Chanah when she was praying for a child? Did the letters of the Choshen illuminate on their own, without the request of the King or the Cohen Gadol? You may recall that when Chanah was praying a certain way to HaShem for a child, certain letters illuminated. At first, Eli the Cohane misinterpreted them to read "Shikur" (drunk) leading him to think that Chanah was a drunk and later the same letters were correctly read by Eli as "Kosher" indicating her method of davening, as commonly practiced today, was desirable and kosher (acceptable) to HaShem. Was it common practice for a Cohen Gadol to routinely inquire of HaShem via the Choshen for matters such as a woman davening or did Chanah ask Eli for a decision from HaShem via the Choshen, but again that doesn’t concern either Judgement or National importance?
According to the Gr"a (the Gaon Rabbi Eliyahu of Vilna) the Halacha that the Choshen could be consulted only for matters of national importance applied to the Melech (king) and Av Bet Din (head of the Sanhedrin). However, the Cohen Gadol who had access to the secrets of the Torah could ask the Choshen questions for personal Torah understanding. (This is based on Ramban Shemot 28;30). Eli knew Chana was a righteous woman and wanted to understand how she could be drunk, so he asked the Choshen. (see Rav Yitchak haLevi's sefer on Shmuel, Parshiot B'sifrei Haneviim, page 21).