Is it permissible for an educational and career counselor (30 years old) for high school students (15-19 years old) to include religious guidance (such as sincerity, halal sustenance, آداب الاختلاط (etiquette of gender mixing), and fatwas) in their counseling sessions, and what are the regulations for that?

The questioner has done well in being keen to support his consultations with شرعي (Islamic legal) matters, due to their importance and necessity. This falls under giving advice to every Muslim, as the Prophet (peace be upon him) said: "Religion is naseeha (sincere advice)." As for the controls, their essence lies in: considering the situation of the one seeking advice, and the advisor confining himself to the limits of his knowledge, so he does not speak about what he is not proficient in.