I have trouble deciding which tense to use

Here is the trajectory of my story:

I had been told about Carl (Bud) when I was perhaps 6, but I was 55 when I finally learned the fate of my uncle Carl whom I had never met, but I knew that he died in WW II.  In the next 10 years I researched his life through reading survivors accounts, poring over materials in various cities in Montana, meeting Carl’s buddies in Florida, and making friends with a French diver who had visited and photographed the doomed ship in the English Channel.  At last I pieced together and narrated the last of Carl’s 21-year life.  

Briefly, Carl lived in Kalispell Montana, then went to college in Missoula.  He suddenly had to leave his fraternity in Missoula and travel to Butte for induction into the US Army.  After boot camp he took a difficult examination for the Army Special Training Program and was assigned to the University of North Dakota in Grand Forks.  He spent about 3 or 4 months there before the A.S.T.P. was dissolved.  

Carl found himself assigned a private in the infantry, at camps in Arkansas and Alabama for more training with the 66th Infantry Division.  After almost a year of marching and bivouacs that included a final train trip back to Montana for leave, the Division took the train, first class, to New York, then to England aboard the USS George Washington.  

From there the story gets more fleshed out when Carl and his buddies arrived in England.  I have copies of letters his friends wrote.

The stay in England seemed peculiar to Carl and his friends, but the got used to the straw mattresses and the people with their English accents.  Carl was close with his friends, had been for a year or more.  They lived in a brick barracks at Camp Piddlehinton near Dorchester until December 23, 1944, when they were ordered to abandon the camp and march most of the night to a train that took them to Southampton.  They waited hours and then boarded a ship.  Then off the ship.  Wrong ship.  Then onto a rusty ship, the SS Leopoldville.  

The story gets even more detailed because so many accounts exist.  I here change to the present tense.

Carl and his Company E are assigned to a low compartment beneath the water line on the starboard side.  The story soon pretty much ends for Carl because, exhausted and perhaps seasick, he sleeps all that day, weary from the march the night before.  Then the torpedo ends Carl’s life, ultimately along with 762 others.

The story of German submarine U-486 intersects the story of Carl and the SS Leopoldville at 6 pm Christmas Eve.

At 8 pm, when the Leo sinks to the bottom of the Channel, the few remaining men of Company E are already en route to Cherbourg, rescued by the HMS Brilliant.  Soon they meet up on the docks and an experience with some generous black US soldiers inspires several to become very religious Christians.  (These 2 men jumped from a great height to the deck of the Brilliant to safety.)

Weeks later the 66th Division travel to 2 Nazi submarine bunkers on the Bay of Biscay where they meet up with other soldiers assigned to contain the Germans.  At the war’s end Carl’s buddies became more or less successful adults.  Several of Carl’s friends were traumatized because they were aboard the SS Leo.  The others not so much if they went to France that Christmas Eve aboard the HMS Cheshire, a companion ship that carried about half of the 66th Division.

I want to leave the story of Carl and his buddies to posterity, so I struggle with writing it.