i think your confusion is that you cant UNCHECK the boxes marked "trim left" and "trim right", but that is misleading because the reason you cant change those checkboxes is because the PARENT checkbox of them is already disabled.
So its a bit of a confusion with the interface, but trimming is turned off when the parent group checkbox is unchecked, so there is nothing to worry about.
ps.
good work figuring out how to do what you wanted -- the preset format things can be tricky to get the grasp of initially.