So... Imma summarize this as best as I can (may the blue penguin who lurks the comments forgive me):
Grandpa married an Assassin of the Order
Grandpa appears to have adopted and raised the original "Doktah" in the Arknights dimension.
Grandpa and Apprentice part ways, Grandpa's Wife passes away, moves away to another world and grows old as a Fishmonger.
Becomes friends with a small blue penguin admiral / commander in charge of the nearby naval base and land-based forces in the area he was living in.
After a brush encounter with the shipgirls (and passing on the Assassins vision to Shigure), Apprentice comes and finds Grandpa pleading for him to take over at Rhodes Island because of his ill health.
Grandpa agrees and moves to begin the duties as a doctor in his apprentice's place, apprentice dies, taking with him the baggage of the Arknights universe's particular story beats in direct relation with him to the grave, leaving Grandpa to do what he can from there.
(I had a different summary but I deleted that comment because I was being dum.)