Depending upon when you go, the cadre may be working on finishing up graduation and orders for those midshipmen heading off to summer training. But you can always request an appointment and I am sure someone would help you out. As we get into the summer, the cadre may be on leave or away from school for other reasons. But there should always be someone that can answer questions. If you go during the school year, you can ask to speak with current students and even attend some classes (shadowing them).
No need for a tie. Appropriate civilian attire for midshipmen is khaki's (don't wear jeans) and a collared shirt (dress or polo). If you match that attire you should be fine.
This interview will have no bearing on any future application you submit. It is a great chance, however, to ask any questions on NROTC scholarships, college side-load scholarships, unit training, life as a midshipman, etc.