Love and the Soul are the two most complex topics in philosophy, it's not that this series tried and failed, it's that they tried at all.
Re: Sacrifice; in re Spike. The Divine beings in the Buffy verse, Glory, Jasmine, Illyria, they all have a way of instilling awe and love in people. Perhaps the same is true of the Key and her Scooby cult? Spike can love Dawn because she's the Key, and can love Buffy because she's "Summer's blood?" Something to ruminate.
The Moira Blood being consumed by a non-ensouled vampire has never been explored. They call themselves "Dark Warriors," which frankly, doesn't make any sense. My head canon is, the Ether spoken of in the Ritual of Restoration is some sort of limbo you stay in when you're soul/life is being theived by an undead entity, you are still tethered, and the Moira Blood would recall you and purify the demonic entity. I don't think vamps would become non-responsive soulless entities, waiting for something to take possession.