Trevor,
as we have seen Heidegger's being-with is what we now call a communitarian ontology. He simply denies the presuppositions of individualism (of Husserl and Sartre following Descartes), in which the solitary individual subsequently bridges the chasm between itself and others through social involvement. We start from a shared public world.

So Trevor in Rostrevor is embedded in a local mode of this public world (his friends, painting, music books etc) and its emotional and ethical connections to others. These come to the fore and achieve prominence as care. Human beings care because they are already involved in the world and its meanings.

I guess you call it Hegel's ethical life, or Sittlichkeit, which means customary morality and refers to a way of living rather than a theory.

Sittlichkeit presupposes that society is just particular beings involved in certain sorts of complex relationships. Of course, Hegel's triadic constitution of ethical life (family, civil society and state) is far more complex than Heidegger's. What they have in common is the way of living in a social order and the critical reflection on that customary morality.

Given the communitarian ontology I find the interpretation of Heidegger as an individualist suprising. This interpretation appears to get its hook from Heidegger's conception of authentic life that resists the perspective of the 'they' which is the predominant mode of being in everydayness. The every day refers what is normal, customary and habitual involved in our day-to-day existence coping. Inauthenticity is a losing of the self into the conventional, ie., the normal routines of social existence.

Authentic existence is a breaking free from the conformism and conventions of the everyday that we are thrown into. Then individualism is up and running.
May not this be a recognition of the truth of Rousseau's - and Kant's - moral position - the concept of an autonomous subject whose essential freedom consists in not being forced to accept anything as valid unless his conscience, will and reason have given consent to it?