For the star shaped locked:
<span class='standouttext'>Spoiler : </span><span class='spoiler'> First go to the room where you did the test with the golem and the imp.
There you'll find a raanaar corpse. Click on the corpse to find a pickaxe.
Equip the pickaxe. Now carefully examine the wall on the north side in this room. There's a part that says 'broken wall'. Click on it with the char that has the pickaxe equipped.
The wall disappears, you can explore this area (beware, because as soon as you enter it, some ghosts will attack you). Now go to the east. In the eastern corner there is a water pool. You'll find a star amulet that is basically the key for the star-shaped lock. You don't need to equip it, just keep it in your inventory.</span>
For the three doors:
I'm not really sure which doors you're talking about. If you're talking about the three doors right next to the other, well you can't get there.
General opinion is that this is a case for lockpick (which, unfortunately, is broken). There's nothing important there anyway.
Is that chest you're talking about the stone/silver chest perhaps?
Well, no, you can't open that yet. It's part of a nice little chest odysee (which will send you running all over the map), that you can only start once you've found the golden key in the academy level 4.
and last:
I know the raanaar instructor tells you you won't be able to return to the lower levels, but he's plain out lying. You can return just the same.
So don't worry too much now, you WILL be able to return.