The problem is not with him being a vampire for X years but the idea that some vampire hunter knows where he is even though he is in the middle of nowhere and appeared there just few hours ago.
Correct. I wish I had worded it that well.
Even if they just went with a simple scenario adjustment that you hear about from the hunter himself, it would add realism to the situation. If Astarion's "hunt" began a week prior to the events of the game, that would explain why hunters are already scattered around looking for him and why someone might already be at the hags lair asking for advice on where to find him. But in this scenario, it wasn't the abduction that freed him. Rather Cas freed him a week ago and he just happened to get abducted shortly after.