The dealership isn't allowed to open the transmission as directed, but didn't know they received codes for the solenoids. Before you change solenoids, a fluid and filter service may clear up those codes. The valve body may just have clogged passage ways that prevent the solenoid from diverting pressure. But it seems like your current course of action should alleviate the problem. Fluid wise, you should not need anything more than a Dexron VI synthetic.