@Darkariel:
The guy that probably gave the order to Buster Call might be Spandine
(This would make a reason why Kuzan the Ao Kiji entrusted Spandam with the same power) this if Spandine is in fact Spandam's father
The Buster Call is probably already in motion; that's probably the reason the Navy attempted to contact the Cipher Pol ship as well as the reason that Kuzan was alerted that CP9 had reached Ohara. They were attempting to tell him that there was no point in him going there.
Something has changed the World Government's priorities since Spandine was dispatched so that simply rounding up the various scholars and their research is no longer enough; most likely it was Olvia.
Sengoku is Gensui and Gensui is a Fleet Admiral patent not like the other three that are Taishou = Admiral
Again, he's really an Admiral. There is no rank insignia for anything above an Admiral.
Let me explain it this way (it's easier with an army and it's star system), the Japanese army during WWII had three levels of General: Lieutenant General, Major General, and General. A General wore three stars while a Gensui wore three stars, because they're still a General. It's not like the American system where a four star General can become a five star General of the Army during wartime.
A Gensui will wear identical rank insignia to an Admiral or General because they still are those ranks, but have been singled out because of their actions. It's translated into other languages as the highest equivalent of their hierarchy, but it doesn't really work like that.
I call him a Fleet Admiral myself for the sake of convenience, but that's not technically correct.
Gensui = Fleet Admiral, so Sengoku is the main leader of the Marines in charge if he dies one other would take is patent (possibly nomeated by the Gorousei) and maybe it could be one of the three Admirals
Not necessarily, Gensui is not a rank that belongs to the military hierarchy. Really, it's a title more than a rank. Just because Sengoku isn't around does not mean that the Gorosei or whoever will appoint another Gensui.
Sengoku doesn't outrank the other Admirals because he's still an Admiral himself, but he's in charge because of the extra moral authority granted to him by the title of Gensui.