What is the difference between mindfulness and concentration?
Concentration is part of every mind state: the normal, regular, every-day-of-life mind states; the unwholesome ones; the wholesome ones.
It is absolutely not limited to jhana states!

One-pointed means basically that it focusses the entire mind on to an object. In vipassana we speak of momentary one-pointedness where the mind in this moment focusses entirely on this object and the next moment on the next object.