Why would Akainu be kept in the dark about Dragon being a former Admiral? If Dragon truly is a former Admiral, who said Akainu doesn't know?
They want to cover-up the identity of the worst criminal in the world?
Nothing indicating him being part of the marines, during Roger's execution.
You must be registered for see images
And then you skip 10 years ahead, hes shown to have already gathered numerous strong people, to join his army (when he rescued Gray terminal).
I highly highly doubt he was an admiral in those 10 years , between the time of roger's execution and the time of him saving Sabo.