Sound explanation. o3o
Ansem and Xemnas are regarded as outside time but despite my own boredom theories on it and others that comment can be taken a couple ways. Because to travel in time in any direction is technically to be outside it's binding flow. It's very likely and possible they to are just past selves like the rest.
The Realm of Sleep itself is also outside time with no true flow of it and since Ansem and Xemnas were there with YX they were also technically outside it's flow then too.
Who the other Riku was has been a thing of mystery for some time as you've mentioned. There's a couple possibilities of who though. Well only two really.
There's KH1 Riku. In Coded you get an extra scene of the past showing Ansem, in his Robed Figure state, leading KH1 Riku away in a corridor. Meaning it's possible this Riku is from KH1 either after Ansem steals the body or at some point before.
The fact that Ansem didn't kick Riku out of his body until after Sora uses the Keyblade of Heart to release his heart pretty much leaves all of KH1 as room for that Riku to appear.
But it could also be Repliku. Nomura could always pull the bullshit that the darkness enveloping him as he was dying was really just him pulling an Axel and disappearing/running away.
Personally I hope it's the former though cause to bring back Repliku after CoM would ruin his character. His death and overall story was impacting, it makes you think and his death has a big part of that. Take it away and you kill that story.
EDIT:
And a good thing to remember about the eyes is that while it's clearly a hint of Xehanort not all his selves have them.
Ansem & Xemnas have orange tinted eyes and Apprentice has brown.
Ansem and Xemnas are regarded as outside time but despite my own boredom theories on it and others that comment can be taken a couple ways. Because to travel in time in any direction is technically to be outside it's binding flow. It's very likely and possible they to are just past selves like the rest.
The Realm of Sleep itself is also outside time with no true flow of it and since Ansem and Xemnas were there with YX they were also technically outside it's flow then too.
Who the other Riku was has been a thing of mystery for some time as you've mentioned. There's a couple possibilities of who though. Well only two really.
There's KH1 Riku. In Coded you get an extra scene of the past showing Ansem, in his Robed Figure state, leading KH1 Riku away in a corridor. Meaning it's possible this Riku is from KH1 either after Ansem steals the body or at some point before.
The fact that Ansem didn't kick Riku out of his body until after Sora uses the Keyblade of Heart to release his heart pretty much leaves all of KH1 as room for that Riku to appear.
But it could also be Repliku. Nomura could always pull the bullshit that the darkness enveloping him as he was dying was really just him pulling an Axel and disappearing/running away.
Personally I hope it's the former though cause to bring back Repliku after CoM would ruin his character. His death and overall story was impacting, it makes you think and his death has a big part of that. Take it away and you kill that story.
EDIT:
And a good thing to remember about the eyes is that while it's clearly a hint of Xehanort not all his selves have them.
Ansem & Xemnas have orange tinted eyes and Apprentice has brown.
Spoiler