IF you ask me, Revan beat Malak, who was the currently accept Sith Lord, who was the former apprentice of Revan. Even if Malak did beat Revan, he could not kill him, and Revan comes back and kills him instead. Now, we can argue whether the fact that revan was still alive AND did kill Malak may or may not mean that he never lost the titile of the Sith Lord, that's very much debatable.
However, if you are DS, I think it would obvious that other Siths will either:
a) accept (in face, anyway)
Now, considering that, and 5 years, ANYTHING can happen.
Now the LS ending is harder to explain. Except that the attack on the star forge really costed the Republic, indeed other Sith outside the star forge did exists, and Revan could not continued his/her fault against the Sith somehow and for whatever reasons disappeared with Carth/Bastila.
AS you might now see, with 5 years, and creative thinking, anything can happen.
Unfortunately, I don't have any. :D