1.Where the thermal pad was, and on the two small amp chips (under the metal cover), you need to apply a generous blob of heat sink compound. Otherwise, you might not bridge the gap to the metal cover.
In fact, after applying the compound, you should fit things together, then take them apart again and make sure you get good contact on both sides. Just be a bit careful when pulling things apart, since sometimes the goop will stretch apart like gum and then fall down again where you don't want it to be.
2. Instead of applying all the goop at once and then assembling, I think it's less messy to apply the goop and then assemble as you go:
- put the goop on the bottom, then assemble the bottom board to the case
- put the goop onto the two small amp chips and part of large top chip, then assemble the small metal cover on them (bend the tabs back)
- attach the top board to the bottom board
- put the goop onto the metal cover and rest of the top chips, then assemble the top lid
3. It's interesting that your wifi module had a thermal pad on it (at the bottom). Mine did not. I think DJI is slowly learning.
The way you slid that paper around on the carpet made me nervous: that's exactly how you can generate (potentially damaging) static electricity.