I think that souls and constructs are such an important part of his series, that I doubt that RJ didn't exactly know how it worked.

I also don't see a inconsistency between saying that a soul can be permanently corrupted and that souls are immortal. Perhaps the souls of channelers or other special souls have an inherent protection against too much corruption, so that e.g. Rand's or Birgitte's soul couldn't be used to create Trollocs.