No, two people grappling in Pathfinder occupy separate spaces (this is a change from 3.5). If the person grappled wasn't adjacent to the grappler, the grappled person is moved to an adjacent space, but not to the same space as the grappler.
right. note that you can move thru allies' squares, and them being grappled doesn't change that, so the fighter could move thru the rogue's square, they just aren't supposed to end their turn in the same space. you can even spring attack thru allies' squares, including attacking from the ally's space.
I've only seen one spell and one feat that allow you to end your turn in an enemies square. They are very, very rare. Tiny or smaller creatures must enter player/enemy squares to attack them. This movement usually provokes AoOs.
Monkey Shine is the feat.