Well, you can, but the results might not be what you want or expected. When you paint, the surface must be just right. Sometimes that might mean sanding a little to roughen the surface so the new paint sticks better, or sanding also takes off any gloss on the old surface, because gloss is shiny and doesn't let stuff stick to it well.

Paint and primer in one can is more of a gimmick than really useful. Doesn't mean it won't work. It's just that it's more stable to use separate primer and paint.

It's like shampoo and creme rinse in one bottle. I've tried Pert Plus that had that when it first came out, and didn't really see anything special about it. Besides, 98% of population feels separate shampoo and rinse are best, too.

But doesn't mean paint & primer combined won't work. but just make sure the surface you're putting it on is in good condition.

Not really shiny, and should have a little but of ruff surface so the primer and paint will stick better. All of this is prep work, so the paint and primer WILL work for you.

If the wall is already painted, sure. If it is just bare drywall don't be surprised if you need several coats to cover. P&P is overhyped if you ask me.

If it's a good quality brand then you don't have to prime first.