Can I rely on the events in iCalendar being ordered by start date descending? I checked the iCalendar RFCs but I don't see them talking about how the VEVENTs are to be ordered.
It may just be a coincidence that my Google test calendar lists them by order of appearance oldest to newest.
No you can’t rely on the order of events. I've seen a variety of ordering from different applications. If it's not specified as you have found, developers will do all sorts, also StartDates can be awkward eg: recurring events with instance modifications.
Also note that the full picture of an event may consist of multiple VEVENTS. Eg a recurring event may have one VEVENT with the RRULE, then modifications of instances of the RRULE may be also in the file with same UID, but also an updated SEQUENCE, and RECURRENCE-ID and probably also LAST-MODIFIED
Theoretically even a normal Non recurring event could have another VEVENT modifier in the file, although I don’t think I’ve seen an application issue that. Usually they drop off the outdated VEVENT if it is completely replaced by a modified one.