You’re not buying the conventional self conception? Ultimately no self, conventionally, we label the 5 aggregates as self.
Self doesn’t need to exist for electrons for there to be separate electrons in 2 places.
No self doesn’t deny individuality.
5 aggregates have feelings, therefore morality works. Kamma “doer” is the volitional formations, kamma “bearer/sufferer” is the other 4 aggregates.
Kamma being impersonal law still works for past kamma results happening to arahants, but arahants having eradicated all sense of self cannot create new kamma, but since they are also freed from all roots of evil, we need not worry about arahants having no justification to do good, avoid evil (because cannot create bad kamma), because arahants are incapable of doing any evil deeds, on purpose.
I don’t get your link between your view of self and the attainment of cessation of perception and feeling.
For spreading metta, since you already accepted that impermanence means dukkha, so too the metta spreading is dukkha due to it being impermanent. And compared to Nibbāna as you pointed out at the end.