Summary:
The query of זנוזל"ח is when one already used the property of another, but not when one is asked to initially relinquish his (potential) rights. זה נהנה וזה חסר is even if the חסר in not a direct result of the נהנה (but preceded it).
The reason is for it is the ניקף; however if it was the מקיף he is פטור, etc. - טעמא דניקף הא מקיף פטור
Summary:
The query regarding זנוזל"ח is only where there is no indication that the נהנה is willing to pay; otherwise the נהנה must pay the full amount of the benefit.
Provided that he placed it upon the skylight - והוא שהניחה על פי ארובה
Summary:
According to רש"י it depends whether it is part of the building (יש מעילה), or not part of the building (אין מעילה). According to תוספות it depends whether there was a change in the stone (יש מעילה), or not (אין מעילה). A שינוי החוזר לברייתו is not considered a שינוי and is not קונה.