[@Sodium]
How did Jimmy Jameson learn magecraft? 
Why would the torture subject confess to, out of all things, the fact that the Church sent one of their many members to a far East country to supervise some random backwater ritual? 
Keep in mind that the Church is huge and obscenely powerful enough that Billy's organization won't be much of an actual blip on their radar.
His Church sending meager amounts of magical energy through a long-range Mystic Code won't be enough to maintain a Servant, there's just not enough energy getting moved; I recommend homunculus batteries as an economical and inhumane alternative. 
A second-generation magus whose dad didn't seem to do selective breeding with someone who also had Circuits shouldn't have A rank quality. 
Skillset and guns are fine. 

I'll review Musashi once [@IncredibleBee] posts an edited format.