Probably the former. The overall nature of Amestris government was far less malevolent than the manga series'. Considering everyone that was involved in the assault at central were allowed to live peacefully (or in the case of Izumi, dies peacefully) and Mustang's exile was self imposed, I'd imagine Grumman was already high in the chain of command, and just ended up taking over to clean up the mess and do the cover up story or something.
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