Is the following true: a) meditation is a state where you can focus on something with help of imagination/(other perceptions)
No, because you can do that right now: you can imagine easily at any time.
We use the word “meditation” with its ancient, clear, and specific meaning: Meditation is a state of consciousness that is unconditioned, meaning that it is the natural state of the consciousness.
The natural state of the consciousness is to see beyond physicality and understand reality by perceiving it, but we lost all of that when we became hypnotized by desire and thereby acquired the many defects that condition the consciousness. That is, we left Eden (bliss). If we restore the consciousness to its natural state by eliminating our defects, then we can see clearly and naturally again.
Therefore, in "meditation practice" we are seeking to restore the natural state of the consciousness, even if briefly, so that it can see clearly the situation it is in, and gain the needed understanding to change it.
b) concentration is a skill to aid gaining and maintaining a focus in meditation
Concentration and focus are the same thing. These are not unique to meditation: we use them all day long. In everything you do, you apply concentration to some degree or another.
c) one will gain new knowledge/... in a form of epiphany, ie. striking realisation.
"Epiphany" in this sense is better stated — and more useful — as "comprehension," which is what we need the most. Comprehension is understanding. Again, this can occur at any time, not just when "meditating." Many people have sudden comprehension while washing dishes or brushing teeth.
In a sense, one can improve conditions of hearing Divine and gain skills to perceive Divine clearer, but Divine is always the one who decides when and what to feed to one.
Our vision of reality does not depend on boons from the divine. Our current and future state depends on us, not handouts from divinity. We are the ones who fell into temptation, and to be liberated from the consequences of our actions, we have to change.
All of this is being discussed in a new course that is being recorded now:
http://gnosticteachings.org/download/meditation-essentials-course.html