The 'sphere' around each point of interest is essentially an instance within the game (32 player cap etc). Hence this transition between SC and disengaged flight.
No, the engine can happily drop you out of supercruise at any point in space. You can try it by slowing down to below 200km/s anywhere and pressing the supercruise button. It will place you in an existing instance if there is one at that point or spawn a fresh one. This shows clearly seen when you get intercepted only a few kilometers from your destination. I was ambushed 30km from a mining site once and could fly the rest of the way on conventional drives.