I still don't think this is a prequel to Lucky Mark, but rather some alternate universe. Sure it uses Elena's CG from Lucky Mark, but the fact that MC views her as 10-15 years older version of herself is weird makes it something more of a hallucination, than an unbelievable insight into her future. I'd sooner view it as an easter egg. Like the photo of Anya (sister of LM's MC) on MC's desk near the computer, who would be a little girl if this was a prequel.
And based on the gadgets they have it can't be 10-15 years earlier than Lucky Mark. This Elena may be something like reimagining of the character, rather than being the same Elena. Which also makes me think Kate might be reimagining Katerina's character from LM. Although much nicer this time.
Speaking of Kate - is she probably the girl on the main menu wallpaper? Her hairstyle doesn't look exactly the same, but looks more like it than other girl's. I wonder if that's just a fantasy picture or a foreshadowing of how dark it might get for the girls in the game.