Got some questions regarding the seekers, when did Master Xehanort split his heart for them ? we know Braig and Terra were in BBS period of time, is Braig the first Xehanort-Vessel we know ?
While Isa appeared in BBS looking like normal boy -no signs of Xehanort on him-, by this i could only assume that Isa became one of them in the gap period (between BBS and KH1) and is this the reason for not creating a Nobody out of Master Xehanort ? because his heart left piece by piece ?
Finally, how is possible for Young Xehanort to be one of the seekers? didn't he say he will return to his own time?
When MX did split his heart isn't known or confirmed, although he is obviously doing this multiple times and piece by piece as he's infecting more vessels. Going by the implication that heart "shards" can grow into full hearts I guess it's also safe to assume that the original heart they were split off from can also recuperate over time (which is implied to have been the case with Ven's heart in the four-year period between the BBS Prologue and BBS itself).
Braig was definitely either the first or the second "vessel" (or precisely spoken, the first or second "infected") as the change in his eye color happened sometime between his argument with MX at Radiant Garden where his eyes were still brown and him facing Aqua at the Keyblade Graveyard.
Since Vanitas has the golden Xehanort-eyes too though and he was together with Xehanort for four years before MX even met Braig, he might count as the first Xehanort-seed.
Regardless of anything regarding Xehanort's heart-splitting
Master Xehanort could never have created a Nobody anyways because he never lost his heart to darkness.
Young Xehanort sits on one of the thrones (No. 12, to be precise), so that alone should be evidence enough that he's one of the seekers.
Every seeker coming from a time that isn't the present (aka Ansem SoD and Xemnas) have to return to their own time after a while since they can only exist in the present for a limited time. Frankly, MX also doesn't need more since he only needs the thirteen for his plan to forge the X-blade. Once that is accomplished (or probably thwarted for good by the good guys), any Seeker of Darkness beyond MX himself loses his/her importance in an instant.