I think that a third variable that would influence both a child's attachment and their well-being later in life is an innate personality characterisitc. As the nature vs. nurture debate concludes, personalities and beliefs are influenced both by environment, and by their own genetics. Therefore a child's well-being later in life may not depend on how they attached to their mother as a child, but rather it depends on their own nature. That same nature is what causes them to have a secure attachment as a child, or otherwise. Therefore, there will be a correlation between secure attachment and well-being as an adult, but they are not interdependent.
