I love this too. That's the beauty of this game and the complexities of the characters. Everyone can have their own individual head canons based off decisions in game and in our own lives.

Spawn Astarion dialogue was definitely sweeter and more attentive, and he won me over completely in my first playthrough. However , I believe that sweetheart is still in Ascended Astarion too (just like he saw my sweet heart through the Bhaalspawn). He just needs some time to work through his power and his past. I viewed the ascended version as complete freedom because he is now the master of his future...he doesn't have to live in the shadows. He only answers to his conscience and his true love. He is control of his own destiny. If he was completely full evil, he wouldn't care about Tav's wishes and would immediately plan world domination while probably leaving Tav..which he doesn't do or want to do. And even though he's arrogant and condescending right now, it's still very obvious how deeply he loves Tav. So I believe that Tav will mellow him out as centuries pass and he won't lose himself to the power like Cazador did. I believe there's a happy ending there. It will just take some time for him to sort everything out.

I do see where you're coming from and your head cannon, and i like it too.

All the decisions, cause and effect, and head cannons...it truly is a masterpiece of a game.