Alright, so you've made it this far, no turning back now. Theory/headcanon time. (Note: For simplicity's sake I will be referring to Lucina and Robin as couple here, since thats how it was in my game)
At first glance the Future Past timeline appears to be much like the original future timeline Lucina came from. However, there several major differences that indicate that this timeline is actually very different, and serve as the backbone of this theory.
1) In the main future timeline it was implied that Robin willingly became Grima, or at the very least gave into despair and accepted his role as Grima's vessel . This is not true for the Future Past Robin, who is shown to be actively resisting, implying he did not become Grima willingly.
2) Morgan exists in this timeline regardless of if her parent is in fact a second gen character. Furthermore, she remembers Robin before they became Grima and refers to the other characters of that timeline as having been her friends, implying she wasn't just created by Grima but was actually born in this timeline.
3) Grima is more powerful in this timeline, both in terms of stats and also storywise as he was able to kill Naga.
4) Tiki survives up until the DLC's events, when it is implied the main game's Lucina never met her in her own timeline.
5) After Robin gives her his book, Morgan notices she has another exact copy of it. The interesting thing is, during the Hotspring DLC, Morgan discovers she has two copies that that exact same book, with no memory of where the second copy came from. How interesting, I wonder if that could mean...
Yeah, as I implied by that last point, the theory is as such... the Future Past timeline is in fact the timeline the main game's Morgan comes from. How the heck does this work if Lucina is Morgan's mom you may ask? It works because the Future Past isn't an alternate version of the original future timeline Lucina came from, but rather its an alternate version of the game's main timeline, where Lucina and the others came back in time just like in the main timeline but in the end everyone still failed.
In this timeline, things play out like they do in the game at first. Lucina arrives in the past, with her influence changing the outcome of the war with Plegia, with events matching the game's up to the two year timeskip. However, at this point the timelines diverge, with something changing to delay the war with Valm by seveal years, causing it to occour later just like in the original future timeline. The most common explanation is that in this world Yen'Fey fought back against Valm, delaying events (and leading to the death of this world's Say'ri, making this Yen'fey in fact the version you recruit from Spotpass).
In the end though its not important why the war with Valm is delayed, what is important is that this means there is an extended period of peace. During this time all of the second generation characters are born, with their future version being around during this time. It is also during this time that Robin ends up in a relationship with Lucina, during which time Morgan and her twin brother (thus explaining the two Morgans in the DLC) named Marc (Morgan's name in the Japanese version, works as good headcanon name for male Morgan)
However, the peace does not last. Eventually the war with Valm happens, and with all resistance on his own continent dealt with, Walhart's forces are stronger now. The war is longer and bloodier, but eventually the Shepherds win. Things play out then like the game, Fire Emblem is stolen, and this is where things go wrong. Perhaps Basilio was killed in this timeline, so the gemstones were never swapped out, but either way Robin, despite his stronger ties to the Shepherds due to his amnesia, is forced to kill Chrom and against his will becomes Grima (explaining point 1). With Robin's strength added to that of whats left of Future Robin!Grima's own power and Grima's, the Fell Dragon becomes even stronger now (explaining point 3).
With Chrom dead and the Fire Emblem lost the Shepherds have no way to defeat Grima, with each of them, including Future Lucina and all the others from her timeline, killed one by one in the fighting. Grima!Robin manages to get Morgan and Marc to join him either through intimidation, brainwashing, or both. The future falls into despair, leading to the state we find it in during the Future Past dlc.
But then comes the fate of Morgan, and how she ends up in our timeline. Following her encounter with the Robin from the main timeline and her refusal to keep fighting, she returns to Grima to report what happened. As you could imagine, Grima is not at all happy about this, and decides to kill her. However, before he can do so, Robin, just like he does during the finale of the DLC, gains control of his body long enough to send Morgan out of this timeline just like he will do with the Shepherds. In doing so he erases her memories, leaving her only with a few on him before he turned evil and a few false ones so
that she believes she had been traveling with an uncorrupted version him right before she was sent back, wiping her of the guilt for everything she did while working for Grima and giving her a second chance in a timeline were maybe he won't fail her and everyone else.
This in turn would explain why in Morgan's paralogue it sounded like those Risen were sent specifically to kill her. They would have been sent by Grima of the Future Past timeline once he was in control again to make sure she didn't escape.
The fun part is this would mean that Morgan's existence in the game is as a result of circular causality. Because Morgan appeared in the game's timeline, Robin got to know her and thus when he runs into her past self in Future Past, recognizes her enough to say she remind him of, well, the Morgan he knows. As a result of having met Morgan, he gives her the book, causing Morgan to disobey orders, thus leading to her being sent back in time, thus leading to Robin meeting her, thus meaning he recognizes her in Future Past... you get the idea.