Actually, Xehanort did find Heartless within Hollow Bastion before his experiments. Heartless were present, they were just small in number.
What was it that Yen Sid said? "The Heartless are darkness made real - and darkness yet lingers in every heart. The Heartless are fewer. But while darkness exists in a single heart, it will be difficult to eliminate them."
The mass invasion of the worlds by the Heartless was due to Xehanort and chums, true, and this broke down the barriers between the worlds, making it easier for Heartless to get to them. Nobodies will have existed before (strong hearted people may have even ventured to the Realm of Darkness and met their ends, becoming Nobodies), and due to Yen Sid's knowledge of them, he's likely encountered them before.
No, he didn't. He found the Heartless under the castle immediately
after his experiments caused the test subjects' hearts to collapse. He just didn't associate the two immediately. (See: AR 2-3) Ansem the Wise had mentioned "those taken by the darkness" in AR 1, but Xehanort doesn't seem to have linked the two, and he ought to have known what AtW's intent was.
That the Heartless are difficult to get rid of
now does nothing to prove that they existed in the Realm of Light before Xehanort's experiments (with the obvious exception of those summoned directly from the Realm of Darkness).
I suppose the possibility that someone with a strong heart ventured into the Realm of Darkness and lost it exists, but we can't be sure a Nobody would be created under that circumstance due to the differences between the RoL and the RoD (do people in the RoD have bodies in the same way as people in the RoL do? Aqua's considered to be equally in need of "birth by sleep" as the more obviously bodiless characters are...). But, if that was the case, the Nobodies must have been equally unable to escape as the Heartless were.
(Why were the Heartless unable to escape in the first place? Corridors of Darkness appear to operate in the Dark Realm, considering that Master Xehanort was able to summon Heartless from there and DiZ was able to escape using them... Were Heartless unable to use the Corridors of Darkness before Apprentice Xehanort intervened?)
You forget the heart has to travel through the RoD to get to Kingdom Hearts. A RoL heart can easily become a Heartless just by its journey to Kingdom Hearts. It might not become that way in the RoL. It all depends on the level of Darkness in the heart.
Do you have a source for this, or is it just conjecture?
And, in any case, if a bodiless RoL heart was turned into a Heartless in the RoD, it wouldn't create a Nobody for obvious reasons.
