Also, it seems that agonizing blast is now automatically added to warlocks, giving them effectively an additional invocation slot

Nah, I read this as: the damage from Agonizing Blast is automatically added to the damage from Eldritch Blast, rather than them being separate instances of damage. It affects # of concentration saves made, abilities that deal extra damage when you deal damage, abilities that can be used to halve damage from a single source of damage, etc.