Don’t forget that Grover Cleveland once spanked him on two non-consecutive occasions.
One of the funniest, and obscure, jokes ever.
Also, wasn’t Burns a Pvt. and Abe a Sgt. in the Hellfish episode?
Here, Burns is a Lt. and Abe is obviously at least a Capt.
I, on the other hand, think it all of grandpa’s stories are true. In cartoon land, the timespan doesn’t matter; after all, characters age only when necessary to the plot of a particular episode. Also, if Homer can have had all the various jobs we’ve seen him do, there is no reason why grandpa cannot have had a similarly diverse career.