Correct: According to the theory outlined above, no neutral form could have been a PIE nominative, because the PIE nominative would have been marked willfully a

Correct: According to the theory outlined above, no neutral form could have been a PIE nominative, because the PIE nominative would have been marked willfully acting entities. So the root cause would have been the distinction made for nouns (“can willfully act” – “cannot willfully act”), and this must affect all words with noun-agreement (i.e. adj., det.) or words substituting, deputizing nouns (i.e. pronouns). It is wonderful that this formerly semantic system (if the theory is correct) has left its formal trace over millennia and a range of thousands of miles. Such preservation was likely in so far as otherwise a new suffix-inflectional distinction had to be introduced in order to distinguish the nominative neuter from the accusative neuter; and in the history of the IE languages, suffix-inflectional distinctions are rather removed than built.