I think some of your details are wrong.
1. Yes, it's not his body it's where the ball is.
2. If a WR catches a ball "outside the boundary" of the sidelines. it's a catch where the ball is in relation to the ball and the sideline.
Example. If a WR's BODY is across the goal line but he stretches to catch the ball inside the field of play (the half yard line),
it's spotted at the half yard line.
3. That's not true. The pylon is considered the goal line. If a runner extends the ball on the outside of the pylon it is not a TD.