The weak plot point...
...was Tristan's guilt over the death of his little brother in WW1.
His dad explained the obvious perfectly: "Samuel chose to be a soldier and soldiers die." Not to mention Samuel chose to go on that combat detail while Tristan was visiting the wounded Alfred. In other words, Samuel was a young man who made his own decisions, come what may. It's not like Tristan could hold his hand every minute throughout that war despite his promise to Susannah to protect him.
Tristan was smart enough to realize this so I don't buy his perpetual guilt over Samuel's death. It struck me as an eye-rolling plot contrivance.