I'm one of the people that don't believe Chibiusa created a separate timeline. I don't feel like the Sailor Moon Universe's time acts the same as that of Dragon Ball Z's time plot.
I feel as if time goes in a circle and it's all connected. The only place that exists outside of time would be the gates of time.
Taking the show Lost as an example of wonky Time details(Even though the finale kinda ruins it) but there was a point where characters were in a sense, traveling to the past and future and all over time. In their own lives. They would question, "Well, why didn't I remember meeting you in the past if I'm just meeting you now?"
Simple, it would mess with the time stream.
There would be no point to do anything if everything was already known.
I imagine Uranus, Neptune and Saturn were not allowed to the inner court and went on to carry out their old positions, but could meet Chibiusa again once she met them in the past. They could have been there for her both, but once she started to remember things is when they had to leave for sake of the safety of the timeline of events.
(I imagine Pluto never died in the Silver Millennium, but only in the future. She was then reborn with the others. In this theory, her past self is still at work at the gates, but her reborn self lives on into the future in space with the others so as not to be found out and ruin the time stream of events.)
How can this theory work, if they already know that Nemesis would attack etc, they should have just prevented it no?
No, because they probably knew very little details.
I am 30 years old, but I've been forgetful since I was a kid, and as more time passes, it becomes easier to forget.
Even though they don't look old, Chibiusa is already over 900 years old, making the senshi and Mamoru even older.
They can forget the details of a lot of things, and then there is the physics of it, which in their world acts more like magic.
Just like they can force someone to forget(Luna Mind Meld among other things), Brainwashing, or restart their life and have them forget, the universe is just made that way so the timeline doesn't separate and continues in a circle.
Also, if they didn't remember any details, I am sure Rei's visions or Michiru's mirror would alert to them on what steps to take. They have a sixth sense about things.
That's my take on it anyway.