Well I found a way around this problem. Automator’s Calendar.app actions can basically do what I wanted to do (although it’s less flexible), and that doesn’t rely on the iffy JXA/Objective-C bridge.
(Automator uses EventKit to grab calendar data, which is faster/better than the Calendar.app application instance for JXA. I think you actually can access the EventKit framework through the Objective-C bridge with JXA, but that takes me into very unknown territory. Since Automator uses EventKit, you can also use Automator to retrieve/manipulate Reminders.app data, so that’s what I’d recommend for anyone trying to integrate with Reminders.app in the near future.)
And here’s the script I promised: 'Magic' daily planning script