Marie Curie is shown to have a French accent. Even though she married a Frenchman and lived in France for the majority of her adult life, she was born in Poland and would have had a Polish accent, not a French one.
When the soldier arrives and talks to Lucy and is eventually shot by her a little later, he is carrying a Lee-Enfield No. 4 Mk. I, which was not developed before the early 1930s. In WW1, British and Empire soldiers used the Short Magazine Lee-Enfield Mk III, which was introduced in 1907.
Marie Curie was from Poland rather than France. She was born in 1867 in Warsaw and married Pierre Curie in 1895. She kept including her maiden name (i.e. Sklodowska-Curie) all life.
When Nicholas is in 2018, he is seen wearing a wedding ring. However, in the early 20th century, most men did not wear wedding rings. It wasn't until WWII that many men started wearing them.
Near the beginning of the episode, Jiya says that they went back in time 2 days ago. Since they have a time machine that can go back to any day, it is not relevant when the others went back; they can go back to the same date.
When Emma uses the defibrillator to revive Nicholas, she leaves his shirt on. As a woman from the 21st century who claims to have "picked up a few things" about nursing, she would know that the charged paddles need to be applied to his bare skin in order to deliver the counter-shock to his heart.