Also, I think that we will only have, at most, one more member of the Senior Council on the Grey Council. Any more than that, and the Greys have a near majority, and would only need to convince one more (probably LtW) of the righteousness of their cause and suddenly they lose so much of the drama about keeping themselves secret.
With the whole of the Council there, and LtW out of the picture, Rashid seems like my most likely culprit. We know he keeps other secrets from the SC after all.
Merlin is right out.
Cristos ditto, he's (they think) already working for the other side.
Mai seems antagonistic enough as is, very much a stickler for protocol.
LtW, as said, was ill and unable to attend.
Martha Liberty, and Rashid are the only others, and I could see both of them being sympathetic, but Rashid has more development.