I think we can knock the 'Doctor's mother' right out of the ballpark - the Doctor's family is all presumed dead before he began his travels- just he and his granddaughter Susan. Now they might still be alive, but frankly that's destroying all the drama of the classic series -which while not so hung up on the lonely Doctor motif, did in fact show, from time to time, that he was a lonely man who missed his lost family and dealt with it by never thinking about them. Susan was the only one alive, and now his daughter Jenny.
(I mean, if his mother was alive, you'd think she'd have made an appearence or three every time her son was named President of Gallifrey
Susan seems most likely - she looked like Susan. Romana, while possible, seems unlikely as she was never the seer type, but the practical clever type. I even thought Romana might be the woman killed in the chamber, but dismissed that as well. If they'd killed off someone as important as Romana, she'd have been named. "Thank you for your opinion, Ex-President Romana!"
That's the same reason I dismiss the council consisting of the immortality seeking coffin people- ex-President Borusa would have been named as the one presenting the seer's Doctor/Master prophecy. (It was just his style to point out such a way to escape death, when others were silent).
Susan was a bit of seer in youth, and I can see her being recalled to fight in the war as it churned on, with her experience driving daleks off 21st century Earth and relationship to the Doctor flagging her to their attention as it dragged on.
Alternately there's just Leela and Ace, who presumably lived their lives on Gallifrey for some time. Tho I have a feeling Gallifrey, as it corrupted itself, may have expelled aliens or sent two such fighters off to deal with battles far away and never recalled them back. They certainly wouldnt have been given a vote tho, as aliens, even if somehow they were back on the doomed planet.
Which makes me wonder if the other dissenter was Andred <g>, but he seemed to be a high council member. Only the woman was dressed differently, no gold on her gown. I hope his identity comes up too, when the Doctor's next story reveals who she was.