The threat was there and Zero remained as a reminder of that threat.
I presume the fight with Galahad? He didn't predict his moves then. Nor did he see in the future. As for Shirley, she was already in the process of dying, you can't make anybody come back to life. Suzaku basically gives up on life everytime until the Geass is set. Self-preservation is over 9000 with that dude after it.
You mean Leuloch.
First of all, he told Schnizel to serve him. With him dead, either the geass will stop or it will cease to work.
Also, no amount of self-perservation desire will give you superpowers. The Knight of One has a power to see into the future and thus predict all of Suzakus moves. Yet suddenly Suzaku defeats him. It was NEVER explained how. Just like half of the stuff in the show.
Lelouch was a freaking magnet for everybody's hatred, everything from world hunger to the dog who sh*t on the pavement was blamed on him. HE was the big bad of the world, and Zero saved it by murdering him.
And? So what if he was hated? So was Hitler. Yet his death didn't see the world united in peace.
And as for the whole thing. The message could have been pre-recorded BEFORE... but yeah, that's head canon. Even so, timeskips are a factor. The time between one scene to the next might not be 'real time' if you know what I'm saying.
The order of the scenes is irrelevant. The timing is.
You have Schnizel leaving the bridge as soon as Leluloch breaks in. Unless Leuloch moves at the speed of light, him beating Schnizel to the escape craft is...highly unlikely. Schnizel took the elivator on a gigantic fortress (a mile tall at least). Leluch was on a foot.