I'm not sure what you mean by "sling strap". I'll assume that's the same as is also called a "neck strap". Basically it's a loop of flexible material attached to your camera that you can carry it by, and large enough to loop around your neck or over your shoulder.
I like taking nature pictures and often hike for miles with a backpack and a camera. If I'm just trying to get somewhere and am not expecting to take pictures, I'll put the camera inside the backpack. But when I'm bumping around more slowly I'll carry the camera by the strap.
Hanging it just around my neck doesn't work while hiking, at least not for me. It bounces around and sways back and forth too much. I usually put it around my neck and over my left shoulder. It's still pretty quick to grab, but it's not bouncing around and in the way as much, and it also a more comfortable way to carry weight. You don't want to hang a few pounds around your neck for hours on end.
Yes, you want to put on the backpack first. If you didn't, the strap for the camera would get stuck underneath it. That would make it hard to grab and bring to up to your eye, and you'd have to take the pack off to take the camera off. If I just want to take the pack off quickly, I'll grab the camera and slide my arm out of the loop so that it's only around my neck just like I was going to take a picture. At that point the pack can come off with the camera still around the neck. Then slide the arm back thru the loop when you're done.
By the way, comfort when carrying a camera like that is about shape, not weight. I used a Nikon F3T for nearly 30 years. When I finally retired that in favor of a D3S, I was worried that it would be harder to carry since it is significantly heavier. It turns out it's the other way around. The large D3S lies flat against my body, making it comfortable. The smaller and lighter F3 would tilt partially down with most lenses attached, so the bottom edge of the camera would dig into my side a bit. I was pleasantly surprised how comfortable the D3S is to carry.