While Libba Bray is undoubtedly an excellent historical researcher, she does let some inaccuracies slip through. (For example, Ling's family having breakfast before going to Mass and receiving communion, which would never happen pre Vatican-II.) The biggest one, unfortunately, is Sam's real name. Though it became more popular among Jews as the century went on, the name Sergei is almost impossible to fathom as the name of a Jewish boy born to Ukrainian parents circa 1910. You would have to tie yourself into knots to explain that being Sam's given name because at the time, it was simply not a name used among Jews. Maybe if the family had stayed in the USSR and Sam had wanted to move to say, Moscow and pass as a gentile, he might have changed his name to Sergei to fit in. But not in the US and not as a birth name. The irony here? A VASTLY more likely birthname for Sam would have been Shmuel — in English, Samuel.
Sam is a 17 year old Ukrainian Jewish teenager in 1927. Considering the number of atrocities that would decimate Ukraine's Jewish population over the coming decades, when Sam would still be a young man . . .
The Reveal that the serum that Jericho's been taking since the first book is made out of the blood of now presumably dead Diviners, most of whom were still quite young. It could easily have been Ling or Henry's blood . . .
Sam, Memphis, Henry, and Isaiah ( though no longer Jericho) are all old enough to fight (and possibly die) in WWII.