A month ago when the season aired I watched two videos trying to explain the ending and haven’t read or watched any since so if what I’m about to propose has already been figured out I apologize.
It is obvious from the ending that Reginald managed to re-write most of the world to be as he wanted, but I think we underestimated how much Allison actually changed. I think when she finished the machines programing she not only arranged for both Claire and Ray to be back in her life, but I think she is the reason the six members of the original umbrella academy + Lila are in the new timeline and retain all there memories. Despite how hostile it got between her and the others near the end it is obvious she still cares for them, so she made sure they’d be in the new reality and that they would also be all healed up, including Luther being alive and no longer being half monkey as that was something that caused him a lot of pain. I think the reason Five is still a kid is because unlike a couple of the others she doesn’t really know what he’d look like as an old man so she could only change what she knew. She also saved Lila because there wasn’t really much hostilities between them and she knew what she meant to Diego.
She did not however save Sloane or any other members of the Sparrow Academy other than Ben because she did not care for them, especially Sloane as she still holds a grudge for her and Luther being an item. This does not however mean that there isn’t a Sloane in this universe, I think the reveal of the second Ben actually proves this, with the exception of Alphonso and Jayme I think Reginald did on some level care for his children, or at the very least felt guilty for what he put them through, so arranged for Sloane, Christopher, Marcus, Ben, Fei, and maybe Alphonso and Jayme to exist in his new world but to have completely regular life. These doppelgangers would have no memory of any of the previous worlds, so sadly even if Luther finds Sloane she will not remember him. I do not know if Reginald would also bring back the other 29 children born in the original timeline, but I do think he would ensure that Allison, Luther, Diego, Klaus, Viktor, Five, and Lila would never have been born to help insure that they would not be a threat in his new world even if they’d have no memories.
My final thoughts sort of unrelated to the above is I think the next season should do a time skip of either 5 or 6 years because besides Klaus and Five all the characters are supposed to be between 29 and 30 but most of the actors are now in their mid to late 30′s (and Five who is supposed to be 13 is actually 18) so doing a skip will help catch the characters up closer to their actors.