Do pads 6,7,8 have to be connected with pad 12 in this layout?
12 is electrically connected to 6 7 8 internally. If you look at the recommended pcb layout on page 10, it has a pad but its not even connected to anything. Neither are 6 7 8. Its just for heat sinking.

Eagle is showing them as connected because they are the same net. It can be ignored, once you take into account the information from the datasheet.