Im trying to visualize the first photo. It looks like the primary drain line slopes up, at least where it ties into the "common drain". Where does the line go that runs off the tee? No vents? It looks like something got really hot on the cabinet (notice the black spot). Did they use a torch to cut it, or was something else going on?

Jerrys right the belt and suspenders approach (cut off switch and drain pan line) are the way to go.