[Mouri twirls his nib pen in his hand, thoughtful. There aren't that many people who could do that]

There aren't that many who could do all that.

Pearce-san? He is a vigilante.
Eide-san would be another option, military training and what not.

...

I'm not accusing either because there's nothing tangible pointing towards any of them

[That is...progress to fix his jumping to conclusions problem, at least?

Aqua would conceivably be in the list but Mouri steadfastly refuses to even consider her a suspect]