As far as he knows and we know, he is still a vampire spawn. The tadpole is arresting everything about it except for the thirst for blood. As far as the monster manual CR and hp, just... ignore that... Larian doesn't seem to be using the standard CR and hp for things.
He either is undead or he is not undead. And if the tadpole changed him to aberration then it should have changed everyone in the party. Being undead means that you are a corpse. You don't eat, breathe, or sleep, you are immune to certain effects like poison and necrotic energy, you are not affected by healing magic, and clerics can attempt to turn you. You could argue that the tadpole removed his sunlight sensitivity to make Astarion stronger, but it actually seems to me like he got nerfed hard.
Idk what to tell you, man. I think you're treating it like Larian is using textbook DnD 5e rules for everything and that's just not the case. Whatever drives the narrative of the game is what they go with. There are just 5e
elements. If you talk to Astarion enough you learn it, anyway. Being a vampire spawn is "none of the powers, all of the hunger" with a bonus of "mentally enslaved to my creator." And all Astarion knows is that while there's a tadpole in his head, he is free of the major drawbacks of his condition at the moment re: sunlight, needing an invitation to enter a home (if you use him to sneak into Ethel's cabin rather than just run the whole party in after talking to her) and being beholden to
. Though he does mention that running water still causes him pain, and that mechanic is included if you take him into the swamp or the rocky beach behind the druid grove.
