A fascinating topic. I would have it, I do believe, that the merging druid would add his soul and intellect to that ancient spirit. Such a merged spirit would be almost wholly subsumed by that great spirit. A well role-played character spirit of the land would by degrees become increasingly less human in personality, and almost certainly more detached from human affairs with each new level he gains. Even at 21st level, such a character I would rule to be immortal.
According to AEFW, even mundane high-level druids may transport themselves to the Elemental Planes should they wish, and surely so could a PC spirit of the land. However, even at 30th level, such beings would have no natural home upon the Elemental Planes. They might venture to such realms, perhaps aiding and meddling in the Silent War in order to effect parallel changes on the Prime Material Plane, but they would never retire there indefinitely. Upon reaching 30th level, their powers and even existence are wholly linked to their guarded lands.
Interestingly, even if their lands were wholly destroyed, I am not certain such a PC spirit of the land would truly and fully die. Review this passage from their MC entry:
Spirits whose resident land feature is de-
stroyed either die or return to the elemental plane, no one is cer-
tain which. This is a mystery even the great druids cannot
answer. Neither does anyone know where the spirits originated.
The popular theory is that Athas was once a world so filled with
life energy that the spirits may have been responsible for the cre-
ation of life itself.
To fully explore your question, Pennarin, I believe this leads us once again into Dark Sun’s cosmogony.
A secular Creation of the Dark Sun universe --that is, creation without gods-- seems all but certain to me. Whatever the act of Creation was, I speculate that at some primordial moment, there was an inherent unity when for but one instant the Prime Material and Ethereal Plane were One. This would explain the profound and essential connection between the PMP and Ethereal Planes. As creation settled, the Ethereal spirits from the beginning of time seem to have settled upon inhabiting the Elements that most appealed to their Nature. Some of these spirits must have settled upon the PMP, and among the greatest of these was that spirit that became the Spirit of Athas itself. This being may have literally helped form the planet, and to it were attracted lesser spirits, the future spirits of the land that inspired the creation of the great land formations of that world.
In theory, should the lands of a spirit of the land be wholly destroyed, a PC spirit might return to the Ethereal Plane from whence it (or at least its predecessors) originated in the depths of primordial time.
It would be fascinating to hear any group of players actually played a PC spirit of the land. As I am sure is the case for almost all of us, none of my players have succeeded in ever reaching such advanced levels of play, and for those of you who would boast of having done so, I would narrow my eyes in suspicion, as you would need to convince me of the quality and maturity of your campaign in which you legitimately reached 21st/21st. How fun if anyone actually ever has!