There's no need to guess like that. Just search for "mass graves gaza", there are plenty of reports of actual mass graves, from Reuters, NYT, SkyNews, The Guardian, etc etc, with pictures and all. See my other comments in this theread for links.
Nope. Those are just buildings. Without people, they are just structures. Again, if Hamas was firing at us from behind civilians (which they do a lot), then we had no choice. You can't just call anything a mass grave for the sake of convenience.
You can’t just call it “collateral damage” for the sake of avoiding the label of genocide while you’re murdering, starving, and barbarically destroying a society of millions of innocent people.
The rubble of the numerous civilian homes and buildings you've leveled are the mass graves.