I would think that if Sheriam was turned, she wouldn't be a reluctant follower. Plus, RJ said that turning someone was complicated to set up, and I don't see why the BA would need Sheriam that much.

As I understood her PoV in TGS, Sheriam joined the BA, because she wanted power. Sheriam also says that she asked too many questions in her past and - in her PoV in PoD - thinks that she should never have talked to a single Sitter. I understand it that way that Sheriam was noticed by a BA Sitter many years ago, who recruited her with the promise of power. And according to Sheriam, she got this power to raise in the WT and punish those, who spited her (Galina similarily says in one of her PoV that she could order the death of a King or Queen), by joining the BA.

Apparently Sheriam just didn't think that she would ever become involved with Tarmon Gai'don, the Chosen and the Dragon Reborn. Since normal BA aren't that powerful, Sheriam might very well have been in the Supreme Council.