18-55mm Nikon back focus issue?

Yes it's normal. A prime lens is usually sharper than a zoom, especially the cheaper kit lens zooms. The difference is because a zoom lens has many more lens elements than a prime lens - and because of that, it's harder to design a zoom that's as sharp as a prime. There are zooms that are optically better than the 18-55mm kit lens, but they are considerably more expensive, since it's more difficult to design and manufacture a sharper one.

By the way, do you know you can fix that minor fuzziness problem you get with a zoom lens. You can simply apply a little extra sharpening in your image editing software - just a touch will do it. You will get a better result if you shoot RAW, and use your RAW processing software to apply the sharpening, rather than trying to sharpen jpegs.

Anyhoo, check it out . . .

Yes, the f1.8 50mm would be sharper than the f3.5 18-55mm. Prime lens are almost always sharper than a zoom (same manufacturer).

Zoom lenses are more complicated to design and build which carries with it more issues. But they still have great value.

Prime lenses are almost always sharper than zooms.