On the whole Inheritance thing: Velioth's scroll made me think that Cazador probably was the "black sheep" in Velioth's family, the one unruly spawn that was not easily dominated but tried to grasp for power himself. So Cazador got the special treatment from Velioth, which maybe in a sick twisted way can be called "tough love". The rules of vampiric existence (Always dominate. Power comes from solitude. Do not act in haste.) are considered valuable lessons for the vampires, so driving them home would make the spawn stronger - either a stronger slave or a worthy heir, should they eventually best you. Velioth seems to have taken his eventual defeat with grace. He seemed proud of Cazador to me.
Cazador and Astarion's relationship seems to have been similar. Astarion seems to only one defying his master, while the other spawn are either daddy-boys like Petras or have their own agendas like Leon or Dal who is looking to cure herself. Act 1 shows that no matter how much Astarion hates Cazador, he does approve of the rule one and two and tries to enact them. The third rule is the one he has problems with, it's also the one that Cazador goads him with at the start of the ritual, where Cazador tries to make him careless. But despite his temper, Astarion has learned that lesson too, he after all does research the ritual in detail and while threatening the spawn in the flophouse, he doesn't actually burn Petras. Early-Game and Ascended Astarion is of course himself but he is very much Cazador's heir in his morals and everything else that he inherits from him - the ritual, the palace, the lifestyle and of course the rules.
If Astarion always wanted that? I don't know, he doesn't seem to remember much of his mortal life and the only thing we can gleam from his early vampiric life is, that he had a soft spot for the sweet ones. But that can mean a lot of things. It might have simply been nice to be with someone none-threatening for a change. On the whole it doesn't really matter, because at the start of the game he has learned his lessons well. But this also means he didn't have much of a choice up until this point, it's basically all he knows.
On the bored spawn-husband. That made me laugh, btw. My impression is, that he treats you like he treats everybody else. Which I really like. As I have written above, Astarion does not seem to like talking about feelings in public. Whenever he wants to talk about feelings, he takes you aside. All other conversations are treated as public or semi-public by the game. After his confession in act 2, he is basically convinced that you will sooner or later dump him because he has not much going on for him besides a whole lot of issues. So he is at least trying to give you the full romance treatment in your conversations as an incentive to stick around.
During the graveyard scene he tells you that he feels seen by you, in other words you understand him. As a consequence he can now stop being a well trained love-monkey around you and just treat you in a more relaxed manner. He now knows you care for him and not just for his flamboyant, charming persona. He is also telling you the more uncomfortable things, like that he is not sure if the two of you did the right thing releasing all the spawn, that he isn't exactly "happy" with how things turned out but that he thinks, he made the right choice. It's good communication. The only thing I do dislike about that dialogue is, that you cannot thank him for the impact he made on your life in return. This might be difficult for a normal Tav but Origins (including Durges) would have reason to thank him for his support during their quests. - I can tell Mystra that Astarion is the reason Gale is still around, but I cannot tell Astarion. ;_;
In any case, I also especially like this slightly more aloof tone he takes with you post non-ascension because it fits the overall bossy tone he has with Gale very well. XD (No, Astarion definitely isn't a damsel in that relationship.) In general he feels more natural after non-ascension to me, since being cagey about his feelings is what is natural to him.
With Ascended Astarion all the highly romantic lines and flourishes about eternal life and being a power couple are performative, they are (to my understanding) intended to spin Tav along by telling them what they want to hear (all those nice words), but also to present an image to the world, one in which you are a power-couple but in which he also is your generous masters. A demonstration of strength, if you will. During the last night alive he reverts back to his I-need-a-cough-drop voice and his preferred manipulation rhetoric "this is what you want, isn't it". Should you try to break up with him after the tadpoles are removed, he tells you how things are, you are lucky to be is special spawn, who is allowed to be at his side while he rules. He now embodies the rules of vampiric existence without regret and finalised his plan from act 1 to make sure Tav never turns from him. If Tav never rebels, Tav will never know. If the collar fits, it doesn't shave. It's a great dark ending - just a bit too real for me to enjoy as a fictional romance.
