No, Luther didn't "prevent" Five from saving TUA. Their very EXISTENCE is what was causing all the apocalypses and splintering the timeline. No matter what Five tried, he would've just made another alternate timeline ad infinitum. Their very births is what caused the original problem. Reggie's "reset" wasn't holding and different timelines were starting to bleed into each other (hence all the "aritfacts" people were finding, and memories of "other lives" that were happening). So, no matter what they tried to do that involved saving their own existence, nothing would've worked. They all had to die to extinguish all the marigold in the various timelines and bring the universe back to the original "correct" timeline. Bummer.