[Mouri closes his eyes. Mettaton is not the kind of person that would allow one to sit back and see stuff happen. If they were going to fight him in some sort of word fight...

...then may as well fire any theory they could think and see what sticks.

It's desperate, but what else could be done]

The wrench was already in the crime scene.
Aqua brought the wrench.
The wrench wasn't the crime weapon.

I concur you wouldn't go all the way to the workshop to get a weapon, but that doesn't mean you didn't kill her. The scenario you are refuting is one of many possibilities.

[It was not much progress, but better say something than not saying anything]