Yes, there are only two Hellspawn left.
I actually suspected arachnidsGrip's role, if she would have a secret one, to be one I know. It acts like the seer, but she can ask the roles of three people in one night, and gets them but doesn't know which role of the three corresponds to which person. When you get three Havenites, you know each of the three are are trustworthy, but can't say their exact roles. This explains why she could know much at once, but couldn't give the exact information.
Still the theory of her being the siren looks quite reasonable too. And as gimmiepie said, charming the seer could indeed give that much information. But there is one problem. This theory needs the siren, arachnidsGrip in this case, to have charmed the seer in the first night, but we were somewhat certain that
Sphealo in fact was charmed in the first night, who appeared to be a Hellspawn. Add to that that
arachnidsGrip herself put the focus on Sphealo too.
There was deduced that Sphealo was a charmed Hellspawn, but according to this theory that is completely unlikely, but she was Hellspawn in the end. That could be coincidental, or this theory could be rejected.