Maybe the beam can be smaller/more focused at lower power? I would imagine it wasn’t on full strength firing at the Rebels. Full power would have been insane at such close range.

I would think the Star Destroyers had them boxed in, and the DS certainly could rotate easily to fire in other directions.
How does any planet destroying super weapon work? It’s a fantasy film, not 2001. As a kid, I presumed the beam pierced the core of the planet and blew it up from within. Looking at freeze frames from the original film, I’m not sure what’s going on.
(Best image I could find on short notice.)

It’s like the beam splits and spreads around the entire planet. (Some fans theorize Alderaan has a planetary sheild that temporarily deflects the beam before it fails.) Can’t find an exact screenshot, but I think the Rebel ships that get hit in Jedi simply explode.

The deleted scene where the Emperor orders Endor blown up is problematic. I don’t think being in orbit of the planet you’re destroying is a safe place to be.