Good news! After spending over a month at Canon service center, my 70D has been fixed. My eyes can tell me that I am now seeing a very big improvement on my photos with regards to their sharpness and consistency. Previously, my camera was very poor on my 24 105L lens. Even at 5.6.
With molyb's analysis on the alignment of the af points to the sensitive areas, I was able to explain the issue to the technical team leader over the phone clearly which I think made him successful on the repair. The first one was a failure due to lack of communication I guess.
When I picked up my 70 this afternoon, I asked them if I can have a chat with their team leader to express my thanks to him and so they called him. I was able to have the chance to ask him about what he did to my camera. First, he confirmed that the focusing areas where way off from the af points we see on the VF. Then told me that no spare parts were replaced (except for the af unit which was replaced on the 1st unsuccessful service). And he continued explaining, he made electrical and mechanical adjustments on the whole af system. According to him, the VF mechanism were not aligned to the af sensor causing the camera to front and back focus randomly and they worked on that. I checked the mid point (again with the use of a bond with a single line drawn in it) and found it centered this time. Even the three af points adjacent to it who were numb before can now focus accurately. I didn't quite understand about the electrical part but it sounds like some spare part is not receiving the proper power to operate correctly.
To cut the story short, they did great for the second time and I thanked them a lot. My new sigma 50 1.4 works well with it and Im happy for that.