I will answer the questions from what I have seen in 30 something years of hunting my area.
When I first started hunting up until the age of 10 all I had was 5 old styrofoam decoys that were painted black. These decoys had patches of the white foam showing through the paint. I used these when I hunted behind my house on the flood water and killed alot of birds over them.
Everyone else I know had nothing but rigs of Mallard decoys and black jugs. That's all we ever hunted over until I was about 15. I then started painting decoys up as other species, but it didn't seem to help any better.
We always killed other species over Mallard decoys and the black decoys.
The only time that placement helps is to put the birds exactly where you want them in the decoy spread. Decoys are used for leverage as well as attraction. Leverage meaning you put the decoys in a certain way to get the ducks to land in the kill zone.
We never saw sleeper and feeder decoys until way late in the game. Never knew they existed until I was about 18 years old. The store bought decoys all had upright heads so that's all we knew. Couldn't tell that it hurt us any at all.
With all of that being said, you should see the spread I run now. Everything I run in my spread has to be realistic looking and the more different poses the better. I have to have species that match the area too.
Now please note that none of this matters. I do this 100% for myself. 99.8% of what we as hunters do is for ourselves. The only things that matters is: 1. Scouting..... being in the right spot. 2. Concelment..... being hid from the birds. Everything else is just fluff.