Here's a little sail for you:
Jerry B mentions sail in FGTW, saying something to the effect that any limb will eventually get straight if it has enough time to free fall. There are a number of factors that will influence how exactly the sail on a large top (or any pieces) will effect it's rotation during the fall. Height is towards the top of the list. Length, momentum of rotation at separation, mass and balance of the piece are all factors that play.
The amount of sail, as determined by the branches left/removed is only one factor. Your statement that " stripping the top out because it causes your top to nose dive " is simplistic and does not account for the many other factors mentioned above.
Newton's Third Law of Motion states that for every action there is an equal and opposite reaction. That is the simplest way to determine the amount of force that the contact with the standing stem had on Billy's top. Any significant force , except a direct downward hit on top of the spar (which clearly didn't happen) would cause a horizontal push on the spar. That horizontal push would create a bending moment that would shake the shit out of the spar. Given the height of that spar, it wouldn't take much to get the it waving around.
The amount of force shaking that spar is equal to the amount of force that changed the direction, momentum and rotation of the spar during the fall. Did we see any big pieces of bark get ripped off the top or the spar? Did we see any movement in the spar after contact?
I had to look at the video in slow motion several times to verify that there was any contact at all. The movement of the spar after impact is almost imperceptible. That is the greatest evidence that the impact had no appreciable effect on the top. Your claim that the impact "stopped the top dead in its tracks" is absolutely ridiculous. You otherwise seem to have a good head on your shoulders but then you make these completely false claims that show that you are either a pathological liar, or have lost touch with reality.
That said: BILLY DID MISS HIS LAY.. the argument for that is simple and unarguable as well. HE HIT THE SHED and hit it pretty hard. So his " BULLSEYE" was bullshit.
So much for a glancing blow Daniel. Can you imagine what that would look like if the camera were squared up with the top? And as the video proves it would have been impossible for "the top to land in line with its trajectory at the beginning of the fall", so upon impact the top bounced over to the right by a couple feet. The top certainly wasn't gonna go through the spar, so being the more movable of the 2 objects it had no choice but to do what it did. Meaning if that spar was not there this top would have landed multiple feet further to the the left than it did.
You ask how the cutting could have been improved
1. Do the job responsibly and safely by simply removing the existing spar before taking that top.
2. Do the job responsibly and safely by simply climbing higher and taking a much smaller top out of the Fir. He was already up there taking all those climb off after all....
3. If he really felt the need to do all that showboating then he needed to make damn sure he made a perfect undercut and a perfect backcut. Truth be told it really wasn't that tight of a lay (8-12ft) Did he misjudge a side lean? Did he simply miss sight his undercut, or did he fucked up and leave some bypass? Only Billy knows the truth.