I think I've stated it on these forums before, but I personally think the Dark Soldier is the Xehanort from Kingdom Hearts I and II(in one of Ansem's reports, he states that he found Xehanort in bad health and with no memory - therefore he took the only name he could think of, 'Xehanort'). Anyway, just wanted to throw that out there for you to think about.
im assuming that you basically think present Xehanort=DS ..cool beens
however, that cannot be possible because during the time when the DS was in existence (meaning during all of BBS when terra and MX and Aqua and VEn were) .. Xehanort was not .. xehanort didnt come to exist until the VERY VERY VERY end of BBS. the ending of the game is what results in his creation .. and we know that the DS is not in existence anymore at that time ..
the DS MAY have played a role in the creation of xehanort (im more inclined to think it was terra and MX) .. but he is not xehanort himself
furthermore .. xehanort was not alive during KH1 or KH2 either so idk what you mean by any of this .. the event you are talking about with ansem finding xehanort was 10 years in the past (right after he was created and the DS ceasing to exist) that is directly after BBS .. and after that .. probly only a few months .. xehanort then splits himself to become ansem and xemnas ..
you seem to be confused about a few things but basically ..
Mx and DS existed during BBS -> after BBS (as a result of it) .. MX and DS are no longer in existence and the PRESENT xehanort is now alive -> a few months afterward .. xehanort is now an apprentice of Ansem the wise .. and he then splits himself to become ansem(H) and xemnas and they are the ones around for the next 10 years until sora comes along
the transition between MX and DS being alive - and then ansem and xemnas coming into existence happens in what im guess to be a matter of months .. all of which happening after BBS so idk
none of what you said seems to make sense or be correct to me