Can I leave an iPhone charging overnight?

Yes, in fact, most people do just that as it is a convenient time to charge. The battery cannot be over-charged. Check this link...

https://www.apple.com/batteries/maximizing-performance/

Yes, absolutely not a problem. You cannot overcharge an iPhone.

Yes. Once the phone reaches a full recharge it will not draw on the power. you can leave it plugged in overnight.