First question: "to quietly leave the building" is a split infinitive - the infinitive is "to leave" and it's split into two by "quietly"
Second question: Greta, Bob, he (not him, it's a subject, not an object), and I (not me - it's a subject, not an object) went to summer school.
Third question: this is false! a parallel structure concerns two similar structures, so manipulating one structure will not change it
