I agree with all your points.
Aiming with the keyboard was a gameplay decision based on the art style we were going for (and the game itself), the game was sort of a "multicrew" game that you play alone, like fixing lights, motors, and such things, we only got to the cannon and submit it that way which was a really intelligent decision as we wouldn't be able to introduce all the mechanics. That said yes, the cannon was based on art style more than how well it fits the gameplay, as it was a secondary minigame we kept as the main drive of the game, we didn't wanna use 360 engine rotations as it ruins the pixelart a lot from my POV (unless it is sort of polished 3D post processing.