As you well know, Noah, virtually no subject is more prone to contentious and contradictory assertions than vocal technique and it is really too complicated to be accommodated by a discussion on this forum. I am not a professional singer but I received some sound and useful training from an excellent teacher in my youth and have spent fifty years listening to and thinking about great
singing. Your instincts and observations are correct; I would say in brief that the lack of squillo is a feature of the absence of proper pharyngeal resonance, insufficient development of both vocal registers by both male and female singers, self-listening, rushing careers and an inadequate understanding of the physiology of the larynx and phonation, resulting in the propagation of silly ideas like "mask resonance". The best way to learn how to sing well is to imitate the greatest singers; they themselves have often been largely self-taught. I cannot, of course, recommend teachers because it is clear that the conservatories are largely failing to turn out the right voices.