Astarion Spawn, in the end, if you ask him to go look for a cure, he lights up with this idea. And here's the ritual, it's a terrible thing, someone says, but what if the new found method is even more terrible, and what? Will the PC, once again, persuade Astarion to stop? And the second time, I think Astarion will no longer obey, because it is really important for him to walk in the sun and other delights, because he repeats a couple of times in conversations: it would be nice to still get strength, but what is done is done, he is clearly upset, suppresses these feelings for the sake of Tav.
That's a very interesting point, it's quite possible spawn Astarion could be offered something like another deal with a devil that would also be an evil act and I agree with you that he is very unlikely to refuse it. In a relationship with spawn Astarion, he has no real power at all and that is just as unhealthy a dynamic as some people are arguing the relationship with A. Astarion is. It won't necessarily end up with one partner abusing the other, but it has the potential. Again, I think you are right, Spawn Astarion seems unable to voice his desires when in a relationship, so for him, being alone is probably healthier because then he is free to make his own decisions.