As for class visits, definitely try to get in touch with 2nd years to get into non-core classes (or first years after the new year). They're very different feel and you'll get better exposure of the full Kellogg experience.
I ended up waiving out of 3 core classes, so I'm actually taking an elective this quarter and will be taking all electives (except FIN I) next quarter. While most students aren't waiving out of more than one or two core classes, if you do waive, you get the opportunity to take a lot more electives in your first year, so it is useful to check in on a non-core class.
At Kellogg, the professors are definitely open to allowing perspectives in most classes. Just let the professor know that you want to sit in before class or arrive with a 2nd year (I did both when I visited Kellogg for interviews last year).