@WhimsiCat
To play devils advocate here....
Its possible that the cannon shell we saw was one of the last big once fired as grapeshots have poor long range and not everyone changes ammo type at once.
Loading a cannon is hard work and they are not going to stop mid reload to swap ammo type.
And yeah they never got super close to the bunkers only close enouth to throw spears but for people who dont have super powers thats not that far off from how it looks here...
Also i'm also fairly sure bunkers are built in a line to not hinder the vision of other bunkers and not in diamond ish formation.
So both are most likely from the artist not knowing modern warfare and how spearthrowing with superpowers work just like you said.
For a manga who have almost too much details its kinda funny in a weird way how little the manga focuses on these details.