Yes, "impossible" is completely wrong. impediat means, like the English, to impede, get in the way, hinder. At the Latin root you'll find the word "foot", implying being tripped or tangled up.
And "accident" also in English implies death, when the Latin casus just means some happening (with usually negative connotation. Comes from the word to "fall". So it's more like the English "befall" (which also has the same negative connotation).
And the phrase "satis sit ad" does not mean will "avail to" but just literally means "will be enough to (with an implied sense of motion) grace and justice", meaning it will be enough to get you through/past the obstacle to the other side (grace and justice).
Interesting that the Catechism does not list the usual three amigos of Baptisms--which, again, one would expect if that's what it ACTUALLY meant as opposed to what people would like for it to mean.