I don't think Billy shares an Elder with Mach. In the chapter where Mach received his instructions to go to America he was talking directly with his Elder and his Master always speak collectively, if it was his agent he sent I would assume he'd say I sent my agent/another agent or something to that effect
|We have agents all across the Americas. Even now they are moving westward to await your arrival. We will instruct one to guide and assist you.|
Also, Billy's fly attack on Alcatraz is directed toward Areop-Enap
|She has released Areop-Enap, the Old Spider. It is a dangerous foe, but not invincible. We have taken steps to neutralize it and ensure Perenelle will remain there until you arrive.|
that said, SC is right either an Elder (Billy's probably) has a score to settle with AE since they know its vulnerability.
That's excellent Hit! If it was Billy What was in those silver bullets then...