Hmm

We already saw the warlord ace took down in his novel, he had the giraffe fruit but we didn't get his name

I really doubt that Oda will introduce somebody this late that Ace, of all people, defeated. He would not be seen as a threat. Also, what Shiryu meant, is an actual Government man, not some shichibukai that would betray the WG the 1st chance they got.

What Shiryu says actually makes perfect sense, hold one of the stones, and nobody reaches the One Piece. But no, there is no chance that it's Doma, besides, we already saw him fight the WG in Marineford.