Generating powers... Something i have been doing wrong...
You certainly do not have to pick all your disciplines beforehand.
"Generating psychic powers", paragraph 3 spells out the order in which you generate a single power:
1) Select a tree.
2) Roll a d6, consult the chart. You now have this power.
3) You may now swap this power for the primaris.
Then, in paragraph 4, you are told the repeat this process for each power you generate. This includes picking a psychic tree each time.
To your other point, yes, you can continue to roll on a tree after picking the primaris. However this will cause an illegal state if psychic focus is obtained with that roll. It's a bit confusing that the "can not swap for the primaris" restriction is applied retroactively after you have obtained psychic focus. It's a terrible rule but it's there.
Last edited by NathanJD; 06-04-14 at 04:40 PM.