I’ve been sitting on these thoughts for a while, and it’s  always surprised me that I’ve never seen them written out by anyone else. Possibly  it’s because my Pluto thoughts skew towards the manga/myu canon than the anime(which is most likely due to my inability to ever get through watching R, since  I default to the anime for almost everything else).

So, with that in mind, I’ve decided to lay out my headcanon
on how Setsuna can exist.

Theoretically, Pluto should be able to experience time in
any order she wants, but canon leads us to believe that she sticks to linear
experience from the moment she is stationed to the moment she leaves the Gate
to save Chibiusa. I think this mostly has to do with how much her duty and her
pride in it is engrained in her; Pluto believes in the rules she was given
absolutely. There’s no reason she can’t leave and travel back in time to the
moment she left, except that leaving the Gate is “taboo.” Not a taboo that
results in death—stopping time is the only one shown to do so—but one she
believes in nonetheless. Queen Serenity gave her these rules, and she will
follow them absolutely, even to the point where she almost kills Sailor Moon on
site. She knows exactly who Sailor
Moon is, there’s no way she doesn’t, but she believes in her duty so fully that
she will eliminate anyone who trespasses.

Except Chibiusa. And Chibiusa’s friends.

There’s no rule Pluto doesn’t break for Chibiusa. She loves
this little girl so much that she sacrifices everything she is for her, first
her duty and then her life.

This is the first time Pluto dies. She survived the fall of
the Silver Millennium. But I think Queen Serenity’s final wish extended to
Pluto, even though she was removed from the battle. So even though she lived
several millennia longer than the others, her soul is still sent to be
reincarnated in the same approximate moment.

And maybe Queen Serenity intended that to happen. Because
since Pluto was sent back in time, rather than forward or being reborn
immediately like Hotaru/Saturn at the end of Infinity, Setsuna is born in a
time when Pluto still lived. Until the 30^th Century, when Pluto leaves
the Time Gate, there is someone
guarding it. Which means Setsuna and the Pluto she is having a thousand years to
be a normal person and a normal senshi.
She gets to have a family and friends, and everything she ever missed.

She’ll have to resume her duty, of course. There must always
be someone guarding the Time Gate, and I think Pluto believes it must always be
her.