Vicious

You are incorrect on Marcus's incarceration. He was imprisoned a few years after E-day for abandoning his post and stealing a Raven to go and save his father, Adam Fenix (who still, unfortunately, gets inducted by Myra). It's clearly shown and played at the start of Gears of War 3 and revealed in the novels. He was a hero after the Pendulum Wars. He messed up shortly after E-day.

Also, they should absolutely include Therons since they were encountered by Baird and his squad during Judgment which covers the very early days of E-day. Hence why when they are introduced in the original, Baird knows exactly what they are. It would be silly not to include them in this new prequel.