I would not follow this video, it complicates things too much. Just follow the guide from here and you are all set: https://gbatemp.net/download/a-definitive-picofly-install-guide.37968/download
And no, you don't need mosfet when you use flex...
Cutting of the shield is not required. You can run the wires carefully through the hole where the fan is and lay the board on top of the metal guard, just place some kapton tape to keep from shorting.