No one is always good all the time or always bad all the time
Yup ...
And that makes them Neutral.
Sorry, I firmly disagree with this point. Even in D&D, only the absolute strictest reading of alignment would make that true. Neutrality implies not caring at all, or caring very little and putting morality beneath some other, probably more personal purpose or goal. You can be a good person and falter in your morality. You can be an evil person and still do something nice. Just look at paladins. They can falter and break their oaths, but that doesn't make them neutral. It makes them flawed and mortal. A basically good person who's stuck in a bad plae in life and ends up having to murder someone doesn't automatically become neutral. They're a good person who's done a bad thing, a terrible thing even. What makes them neutral is that they accept what they did as necessary and decide that they shouldn't agonize over it or over doing it again if they have to. They don't want to kill, but they'll continue to do it for the sake of their own survival and will stop when that's no longer an immediate factor. A good-aligned person in that situation would be wracked with guilt over what happened and would even go out of their way to avoid being in that position again. Or a slightly different example. A morally good person who has a bad temper. Say they get into a drunken brawl and kill someone. They don't become evil or even neutral for that moral failing so long as they feel remorse enough to try and change their ways. If you think that a good person who falters is actually a neutral person (and that is the implication of your claim whatever you explicitly say, if you don't mean that then you should be clearer, a pithy phrase like that is easily misunderstood) then how can you possibly get a corruption arc like you've advocated for?
And regarding Astarion, I think he takes too much pleasure in sadism and petty cruelty for him to not be evil, but I do think that given his circumstances, it's not outrageous that he could be coaxed out of it. He's been the victim of two centuries of trauma, that'll warp anyone, but trauma can be worked through with time and care. I think any redemption from Astarion would require not just romantic love either. Romance or not, what Astarion needs is someone to support him and have his back, yes, but also someone to limit his excesses and lashing out. More than a lover, Astarion needs a friend and role model. And even then, it may well come out to naught.
