"Do I need to replace the LCD screen and touch screen separately?"에 답변I answered this a little more in depth in another question here but no, you don't have to replace the digitizer + screen together. You just have to pry tabs around the edges of screen to work the digitizer apart. That is assuming your digitizer is still good and working.
"Is there an already fused gampad/digitizer assembly?"에 답변Just took my LCD apart. Not sure where this "fused to the screen" business came from (yes, I saw it in the tear down as well). You just pry around the edges of the LCD to get the digitizer off.