""They are going to destroy the planet" in other cut scenes""
That statment was about the Ark crashing into the Earth, not the cannon
"And listing all the calculated feats, we have a [City Busting feat calculated at 6-B which was done with the empowerment with 5 out of 7 Chaos Emeralds. Destroying a large part of the moon calculated at 5-B+ originally in the blog, but recalculated at Low 5-B as seen below in the comments and listed somewhere on DarkAnine's wall that he's okay with that, which was done with 6 out of 7 Chaos Emeralds."
The first feat was a warning shot against the president and again not even at full power, the second the same, and seeing how one emerald boosted the cannon from low 6-B to low 5-B with a warning shot, there's nothing against it increasing to 5-A (the accepted end) with the actual full power
"It was about to be fired and destroy the Earth in Sonic Adventure 2, as well as when Biolizard merges with it to crash onto the Earth"
This is a AoE fallacy, just because they were planning to destroy the planet doesn't mean they can only destroy the planet and nothing more, and the Final Hazard is already 5-A and everything
"And in one route, Black Doom says in the same video where piercing the stars is mentioned,, says he's going to use it and all 7 Chaos Emeralds to destroy humanity"
He never says that he is going to destroy humanity, only he will fulfil the prosperity ritual, which is
revealed in the last story (1:00) that's teleporting the Black Comett to the Earth, not destroy humanity
All in all nothing contradicts the star statment