What is the soul or spirit made of, exactly?

[Peregrinus]

To me, reintegration would mean taking the pieces and putting them back together again to form the same mind/body/spirit complex, placing exactly back where it was, so to speak, on its path of experience.

That would meet the requirements of the term reintegration, yes.

So, if the mind and body are made nonviable, and the spirit disarranged to a point where it can't be reintegrated, doesn't that imply a reset? The entity having to "start over" in 1st density?

We have no basis for making any assumption in implying as to what happens to that energy. It is, as is the next octave, unknown to us at this time.

If it doesn't mean a reset of sorts, what can it mean? Can energy be destroyed?

Of course energy cannot be destroyed, but again, we may assume as much as we like; it is without weight.