In terms of it being "heavily implied" that he didn't summon it, that seems to only come from Belle saying maybe it wasn't him.
Also, I read the tone of what Thaddeus is saying to Dickon while he has Dickon immobilized as being sarcastic. He's condescending and repeatedly laughs at Dickon's efforts. He also never explicitly says his goal is to save her, the line is "Oh, yes, my naive friend. Something is indeed very wrong with Princess Selena. She desperately needs to be saved. I cannot do it, but I'm starting to think you might. Haha!" He also mentions "Maybe you can succeed where so many others have failed." My take is that he's incapable of breaking the curse because he's the one who inflicted it, and that he's tried to use others to break it. I didn't really emphasize the point in my previous post, but Selena being possessed was supposed to be a temporary condition (possibly even accidental, as in he meant to summon the demon directly or be the vessel himself) and that freeing her from possession won't destroy the demon, but merely release it, making the situation even more dangerous.
