Aumaan doesn't actually have the power to destroy the solar systemNohman was just power-tripping. Aware that in spite of his enormous power as the pilot of Anubis, that he would soon die from Metatron poisoning, his narcissism and his madness manifested (like Kuja's from FFIX) into an Omnicidal Maniac urge to destroy everything. The attack would probably have ripped Mars a new one, though.
Aumaan makes black holesAccording to Hawking, Black holes are a matter of density, not mass. Since Metatron can alter space and mass/a give space=density, metatron could convert any given mass into a black hole. If we assume that the entire mass of Phobos was used the black hole would indeed have the potential to destroy the solar system. Eventually. Because a black hole is a function of density and not mass, the actual gravitational pull of the black hole wouldn't be greater than whatever was used to create it. It could take centuries for black hole!Phobos to accumulate enough mass to fall to Mars and even longer to start pulling on other planets. On the other hand, Steven Hawking also says that back holes turn matter into energy, much like a black hole. The resulting explosion wouldn't really destroy the solar system so much as wreck it.
Metatron is eezo, and Nohman was IndoctrinatedAnubis was probably enhanced by Lost Reapertech, leading to Nohman's indoctrination. (Which makes Jehuty's defeat of it all the more impressive.) So when he saw that ADA was a sentient AI... "The Cycle Must Continue."
Metal Gear is a prequel series to ZOEGoing off the design of the latest metal gear from The Phantom Pain, this doesn't seem that far fetched. Add in all of the cameos ZOE get in Metal Gear (including a statue of Jehuty next to other metal gear prototypes on the desk of the man who designed the first Metal Gear) and this actually seems likely.