The Clues to the Twist (spoilers)
So, in my mind, here are the clues that I saw that hinted at Diana being the one who died, and the future just being an imagination.
-The professor looks the same age in both eras
-There isn't much of a sense that these are different times/eras- both Evan's scenes and Uma's scenes feel very much like circa 2007. I thought it was just them being lazy and either not wanting to imagine what 2022 would be like, or depict a realistic 1992, but when I realized that it's really 2007 and Uma's life never happened, it made great sense
-Emma references the Magic School Bus, which is a childhood reference more appropriate to someone Diana's age(if Diana was a 17-year old in 2007)
-The rotting food and dead bird imagery
-"She's Not There" by the Zombies being played
What else hints at the twist?