How to bevel an edge:
1) Define 2 connectors with the same attachment point in the edge. One vector should be poiting in the edge direction and the other is orthogonal, pointing to the exterior bisec.
2) Invoke the Bevel() module (passing the connectors as parameters, as shown in the examples)
3) Use the difference operator
Three examples are provided (along with their STLs)
The code of your part with beveled edges looks like:
bevel(a1, a2, cr = 8, l=10); //-- First beveled edge
bevel(b1, b2, ....); //-- Second beveled edge
The procedure for attaching buttress to you parts is similar, but in this case one of the connector should point to the inside bisector.