The pvc glue would have worked for dwv piping because of the low pressures, but pvc glue is not as "hot" as abs glue. It would have worked fine using abs glue on pvc, but not the other way around. And yes, holding a slope on a trech is not easy. I use a laser. One of the lasers in a hand level works fine if you can do math.

Not responsible for advice not taken...