can't the Executive reschedule whatever it wants unilaterally? I know the DEA has authority to add substances to Sched I, and they add a couple dozen every year.
Yes and no. When it comes to drugs that are on Schedule IV of the Single Convention on Narcotic Drugs (roughly equivalent to Schedule I of the Controlled Substances Act), it would take Congress to overrule that scheduling or it would take the International Narcotics Control Board choosing to reschedule that drug. At present, the INCB has cannabis and cannabis resin on schedule IV and some cannabinoids on schedule I of the convention (equivalent to schedule II of the CSA). This is because the CSA tells the executive to generally follow the INCB recommendations. Unless the INCB reschedules cannabis to a less restrictive schedule, the DEA can't move marihuana to schedule II on its own. If the INCB were to reschedule, the DEA could as well, but would not be obligated to. The INCB schedules under the SCND and the CPS (Convention on Psychotropic Substances.) serves as a floor to the DEA schedules under the CSA. The DEA can be stricter on its own than the INCB, but to be laxer takes an act of Congress.