good theory, thinking further, perhaps remnants are what happens to a displaced mind? after all when Terra's was made xehanort said "Your body submits, your heart succumbs, so why does your mind resist?"
this could mean that xehanort knew what a remnant is; perhaps the status of Terra's remnant circa KH2 was due to the mind in it slowly degrading from being in a non-living shell; likewise the seeming non-intelligence of vanitas's remnant is due to being made from an uncomplete being.
The thing about the mind is that it is still a mystery in the series. The mind is seen in the real world as the conciousness of a person and their will. Obviously the conciousness resides in the heart since MX's heart is what possessed Terra and obviously that was MX's in conciousness in Terra. Therefore it may be likely that the mind is a part of the heart.
We know a complete being in the KH universe is comprised of a body, soul, and heart. The body is the vessel that contains the other two, the soul is the energy or life force of being that keeps the body going, and the heart is the thing that gives emotions, contains memories, light and darkness, and also is the conciousness, will, and identity of the person and as such this may be the mind is part of the heart. After all the heart is very complex and no one in the series understands it completely.
So if Terra's mind was actually in the armor then it would mean his mind/will/conciousness would have been transfered to the armor (Like how the mind/will/conciousness of a person is transfered to the nobody when the heart becomes a heartless) but yet in the secret ending of BBS we see Terra's conciousness speaking with MX's conciousness inside of their merged heart.
So perhaps Terra's a peice of Terra's soul (imprented with his hatred toward MX and some of his memories) was willed into the armor my Terra's heart (which still contained his mind) thus giving the armor life, and Terra's heart/mind from within Terranort was resting MX by controling the armor like it was a puppet (Like how Riku's heart was resisting XH from inside his body in KH1, except Terra's heart/mind was using the armor to help him fight back against MX).
This would explain why Lingering Sentiment stabs his keyblade into the ground and stops fighting after Terranort falls unconcious. With Terranort unconcious and therefore Terra himself unconcious he cannot control the armor like a puppet and as such the armor resorts to a stationary position like it does at the end of Terra's story, where it remains to this day.
