Figured out the issue with the unresponsive attacks.
Seems doing any movement inputs will eat the melee animation. If you hold a direction and press Z, you can hear her start a swing and immediately stop it.
The sword deals damage based on how many frames it stays out, which leads to two other strange bugs:
Lag that causes the game to skip frames can actually stop your swings from dealing any damage. If the animation starts and the frames the sword out are skipped, absolutely no damage is done. Hilariously the opposite occurs with enemy attacks. You can be hit even if you fail to see the entirety of their swing. This bug appears to be the only way for some enemies to survive your swings, thanks to your damage being dealt based on how long the sword stays out for, acting almost more like a lightsaber or fire considering how it works. If the game is running smoothly, basically every enemy is killed instantly.
Conversely, standing inside the tentacle boss (be it the mid-boss or end-stage variant) and doing the horizontal swing causes an instant kill if the entire swing connects. This seems almost intentional because I've managed to repeat it every time I've bothered fighting the thing.
All this being said, I still have no freaking clue what X does.
Makes you wonder what this game would have been like if they had released on the original schedule. Maybe if it had been delayed until December they would even have a fully functioning game.