The Ravenloft style vampire bride / groom theory suggests Tav doesn’t actually need Astarion for those benefits, and Astarion is actually just gaslighting them so they still believe they are trapped. In actuality, they might not be under compulsion at all, nor need to stay close to him. In fact, if that theory is correct, the only binding part of their arrangement would be that if either of them harms or kills the other, they both die.
I'm really curious how the whole dynamic works and I wish the game explained it better, because at this point we can only guess. The distance thing could be true, it could work the same way telepathy works between a true vampire and their bride. He could also be capable of some blood magic manipulation and use these skills on Tav. There is simply not enough information on what the Ascendant can do.
The eyes, among other things was likely from the tadpole, or more specifically the netherese magic imbued tadpoles which blocks out Cazador and messes with other magic stuffs. It also messes with Wryll (and Gale ate a netherese orb even before taking a tadpole to the eye which effed his magic-ing up bad turning them into level 1 punching bags). If anyone paid attention after the non-ascension, all the other spawn lost the eye thing after daddy Caz bites the bullet.
Once the red glow goes away they all get tiefling looking eyes. So Astarion still has unique eyes. Also, if it was the tadpole their look should change once he's free of the worm, but it doesn't happen.
But yes, Astarion should/would know that as long as the Tadpole exists, he could lose control. Tadpole gone, then TAV is his until he decides otherwise.
Oh, he totally knows and viciously lectures Tav about it if they try to break up with him at that point, lol.
