The amount of C4 explosive shown would have caused far more damage than shown to the sedan Trivette was driving.
At the beginning of the show it shows "Huntsville Maximum Security Prison." This is not the Huntville Unit; as the Huntsville Unit is literally in downtown Huntsville. It is surrounded by trees, housing for the guards, and the rest of the city.
Walker handles the shell casing and note without gloves, thereby compromising any fingerprint evidence they could have revealed.