Thus, the SBC agrees with the Roman Catechism that "should any unforeseen accident make it impossible for adults to be washed in the salutary waters, their intention and determination to receive Baptism and their repentance for past sins, will avail them to grace and righteousness."
This is a universally misunderstood/misinterpreted quotation. This does not mean that the desire for Baptism supplies for grace and righteousness, but that the desire would be efficacious in order to obtain the Sacrament (despite the theoretical possibility of an accident intervening). In other words, the desire for the Sacrament would prevail over any accident that might prevent its reception. In other words, it's a restating of the very St. Augustine position that was cited by the OP. So the more appropriate translation is more like, "the desire for the Sacrament would avail them to grace lest an unforeseen accident make it impossible for adults to be washed ..."
There's an analogous quote in St. Fulgentius, where he's explicitly rejecting Baptism of Desire, and he uses the same expression ("avail to") ... probably the Latin
valere in both cases. I'll have to dig it up but I've cited it here before.