I don't understand the thinking behind several parts of your last paragraph - but I am deeply interested in why you think they are so:
I'm not the parent poster, and not religious by any stretch of the imagination, but I'll have a go at giving rational* answers to the questions asked.
- If god is omnipotent / all powerful etc - why do you need to tell others about him? Can he not do this himself if he felt it was the thing to do?
How do you know that He is not? Perhaps he really is speaking through vessels. Due to the existence of free will, others choose to mimic his "chosen" and subvert his message for their own gain. It is your responsibility "to separate the wheat from the chaff", after all.
- If god is generous rewarding etc. - why is there evil in the world>=? Why does he allow situations to occur that turn good people into bad people? (trauma, post-traumatic stress etc.)
Because there is no "good" without "evil". People are given outside influence and are allowed to make choices based on that influence (free will). Lead a "good" life and honor God, you receive eternal reward. If you fall to temptation you suffer the consequence (as it was with Adam and Eve).
- Why heaven - why not just make the real world nice.
It was. Garden of Eden, and all that.
- Why do you believe you know the mind of god? (sorry if I read that wrong - but from your post you seem convinced you do). You may believe that god cannot be mistaken - but do you believe that you cannot be mistaken for thinking you know his mind?
This I can not begin to answer. I certainly don't know the mind of God. I can't know the mind of God. I can't even truly know my own mind.
I am deeply interested in hearing what you have to say on this.
Hopefully my answers were not boring... :o)
*Given the context of an unverifiable story, how rational can any answer based on it be?