Bases loaded. All runners abandon.

NO PITCHES!

These kind of situations are rightly called "trivia" questions.

Could you ever have a balk on this for simulating the pitching motion while not on the rubber or is it always interference?

No, simulating a pitching motion would be starting off the rubber, then beginning a normal windup, stopping and then attempting a pickoff.

The OP indicated the pitcher clearly leagaly disengaged the rubber stepping backwards with his pivot foot, then started to make the play on R3.

It's possible -- if the pitcher went through the motions of stepping off (legal) then taking the rocker step sideways, then replanting the pivot foot, then lifting the free leg to the balance point, then striding to the plate ...

By this time, R3 will have already scored.

If the pitcher sees R3 going and steps off, 99% of the time F1 will just make a "normal" throw to home. And, since the purpose of the balk rule is to protect the runners, and the runner is already headed for home, it needs to be very obvious that it's a balk -- all the benefit of the doubt goes to the pitcher, here.

During the original play the pitcher disengaged backwards, then took a crow hop to a couple of feet in front of the pitchers plate and threw the ball home.

It was obvious that the pitcher wasn't trying to simulate a pitch of any sort.

The only time I've ever called a balk on a pitcher simulating a pitch while not on the rubber was when he assumed a windup position with a runner on first base, stepped off with his pivot foot behind the rubber, but raising his hands up as if starting a windup while stepping off. The runner was fooled into thinking the pitcher was delivering a pitch from the windup, and was quite surprised when he saw the pitcher turn to throw to second. The pitcher was just as surprised when I called the balk before he could complete the throw.

In this scenario, it would make no sense for the pitcher to try and do something similar.