Its nothing to do with singing outside of her range. Because if she can sing them notes properly than they are part of her range. Just because Mariah is an alto does not mean her 'natural range' stops at a certain note. It means that she is most comfortable singing the alto range. However Mariah has a unique voice where she does not really fit any particular vocal catergory. Plus vocal types are not particularly relevant to anyone but classical singer who sing a completely different way.
The deterioration of Mariahs voice is down to irritation of her vocal nodules. After her divorce she stopped nurturing her voice because Tommy Matola made her look after it really well. When she didnt look after it aswell after the Daydream tour it affected her vocal nodules and her voice deteriorated.