1. I assume they can be pretty overwhelmingly accurate, in their field of specialty, or when something important comes through that you need to learn/be told. It may be more blurry on the less important things, or where you need to make important decisions and can go several different ways. A psychic may make an educated guess what path you would probably choose and be able to predict what might wait for you down that road, but in a case where you are or will be undecided and it's 50:50, things become very complicated.
Think of life as a tree. You start at a root, and rise through the trunk. And then there the path branches (literally), and you have to pick. A psychic will normally not see the whole tree but only those branches that appear to be most likely for you to choose. If you later get a different spin on your life and pick a different branch, then the reading will be less congruent with your "new" future.
2. same as 1., but you will have a harder time finding the very best psychics among those who simply charge for image and not skill.
3. No. As a general, statistical statement, not all predictions will come true. It is not possible, because it would be against the principle of free will. Remember the tree. Even if the whole tree exists before you incarnate, consider how complex this tree becomes for our universe. How many things influence your life and the branches you take? How huge must the "universal memory" be to contain all this. No psychic can grasp all this information at once. It is simply not meant to be understood that way from our human perspective. That is for later, when you're no longer bound by time and space, and can kind of look in from the outside.
So, the better the psychic and the better her personal attunement at the specific day of your reading, you may get an incredibly accurate prediction where everything you are told comes true. On a bad day with a bad psychic, you might just as well roll the dice.
This is how I see it, if any psychics disagree, let's hear it from the source. I'm always eager to learn and revise my model of existence.