It really doesn't matter whether the ERG vote for the deal or not, even if Theresa manages to get another vote on it. The DUP is never going to accept it - they see it as damaging their link to Britain, and losing Brexit is no threat to them. Nor is the SNP, for markedly different reasons (they see it as damaging their hopes of getting out of the Union). Which means, without the backing of at least a bunch of Labour or Liberal MPs, it could never pass, even if every single Tory came on board.