I am just wondering about Xehanort's keyblade's design. It looks very ominous. Xehanort's keyblade looks like it has a Heartless, possibly the Guardian that seems to always accompany Ansem Seeker of Darkness.That makes sense. I did think that it was a given that a Nobody would be produced, but this also makes complete sense.
I would also like to argue that, we have seen an instance of where Master Xehanort performed a sort of "Xehanortification", where he perhaps fragments and sends a part of his heart out, and we have seen where he explicitly ejects his own heart out to transfer his being. What he performs on Sora in KH3D is different from the method of transfer he used on Terra in BBS. I'm assuming the same goes for Braig, as after his eyes have become yellow, Master Xehanort is still coexistent with him. He didn't transfer his whole being.
I think the transferring of heart ability that Keyblade Masters attain that you mentioned have more to do with what Master Xehanort did to Braig and what he tried to do to Sora, rather than what happened with Terra. So it's still not exactly clear why a Heartless and Nobody wouldn't of been created at that time.
As for the rest of what you have written, I agree with it on the scope of what we already know, and yes we should base conclusions on the information provided straight out to us, but if you look more at the cryptic things, like the recent teasers for KH3, it raises lots of questions.
The fact that Young Xehanort himself mentions that the Keyblade that Master Xehanort wields is one of the most ancient of all is, just to start, very mysterious. As we know, this Keyblade design shares many similar characteristics to the Keyblades of all the foretellers.
Then we have the dialogue between Young Xehanort and Young Eraqus, where Young Xehanort speaks of a "Lost Master". While it would be natural for Young Xehanort to know about it given that it seems that Eraqus also knew this information, what is interesting is why Young Xehanort brought it up, and why Eraqus was pretending not to know anything. All of this irks of suspicion and has a lot to do with what exactly this man "Xehanort" is. I think it would be quite naive to assume that he is simply a very attained Keyblade Master.
Lastly, in Chi we actually see a cloaked figure, wearing the Organization cloak, speaking among the other 5 Foretellers. They all seem to be listening to him intently. What is said, however, remains hidden. How is this not all suspicious? How can it be that a single person has such foresight and wisdom, being able to a certain extent predict the future? Is it right to simply refer to him as a "Keyblade Master", much less to compare him to the abilities of Eraqus, who was struck down by his own pupil?
Not to mention that this foresight ability also lines up with the Foreteller's text which was also discussed in Chi, and only the Foretellers have access to that...