1) It depends on the situation. How many campers? Is it pretty remote? I'd give as much space as possible/practical. If you are camping at a small remote mountain lake, the is it possible to camp on the other side of the lake? In some cases, terrain may dictate camping in the same general area, and people should understand that.
2)
JUST NO! Unless you own/lease the property, or have some kind of reservation, then you don't own a campsite.
Pack it in, Pack it out. Clean up after yourself.
We always had camp fires when I was younger, but even that is less common now. There was some debate about leaving established camp fire pits vs tearing them down. But in today's society, even the camp fire pits should go away in many remote locations.
I always found it extremely aggravating to hike into a remote area and find cans, garbage, plastic tarps, and other trash left behind by previous campers.
There might be 2 exceptions. Well established camp sites might have a few things like a couple of logs moved to convenient sitting spots. Still, nobody "owns" the campsite. First come...
The second exception might be certain established group sites, for example elk hunting camp sites. They can be a bit of a mess from "developments", but should still be left clean. Again, nobody "owns" it, and I've camped in them during the off-season. But, I might leave the large camp sites to the packers and guides during the hunting season.
Be considerate of people's needs, and perhaps overall impact on the wilderness and forests. So, if simply a place to pitch the tent, then it would go on a first come basis. But, say for livestock or a large group, I might consider moving if I found myself in the only place that would accommodate the livestock or group. That is, if the move is reasonable.