That's why we say "they", could be one group or several acting together.

If it was Israel it was simply another one their 'we're gonna do shit and let you clean up the mess because we own you' shenanigans. Just as Bibi has been doing to Trump since the beginning of his second term. Bibi knows he can do whatever the fuck he wants and Trump will clean up the mess. The only limit is what Trump can't possibly clean up.

The point is somebody had the ability to plant evidence, but only exactly enough to make an impossible to believe story.

There is no need to plant evidence because there will never be an independent investigation. The FBI is probably more corrupt than the government itself. Nothing will come of it. Just as nothing came of the Vegas shooting bullshit story.

Why only exactly as much cover-up as to make an impossible to believe story?

Because a proper coverup needs preparation. It's pretty much impossible to do it properly without any preparation. Not in a public scenario like this.