Oil change is probably the only aspect which does not go by mileage alone, but - like most traditional Honda vehicles - it monitors oil life by some means and lets the user know if the oil needs to be changed. However, Honda has adopted a strategy of tagging on service items when another service is coming due. In most traditional Hondas, the most frequent item is the engine oil & filter change, and then it will tack-on any imminent service item (for instance, tire rotation or replacing air filter, etc.) and move i up for the same service visit, so that you don't have to book multiple appointments.
But in the Clarity, the engine is used so infrequently for many users, that the tire change tends to be the most frequent service item, and so when it gets triggered, it adds on the oil change that is anticipated to be coming up next. It makes little sense to many of us, since in many cases it can be <2000 miles on the engine after 7500 miles on the car. Keep in mind that if you get your tires rotated (either yourself, or at a local garage, particularly for those of us in northern North America when swapping out snow tires), we can reset the maintenance minder (MM) tire rotation. This will then defer the oil change; in most cases for another 7500 miles or several months.
I think your mileage estimates are a little conservative. I think people may report getting a notification on their dash of an upcoming service for tire rotation at 6200 - 7000 miles, but that is a 4 week/1 month warning, so it is really based on ~7500 miles, unless the car is driven aggressively. The oil change, can probably be deferred by resetting the tire rotation on the MM up to 12 months of oil life (when it must be changed), unless the engine is used a lot.
All the numbered MM items are governed by mileage alone, unless you get to the time window (like your example of the brake fluid replacement every 3 years, and the engine oil every 12 months minimum). Honda doesn't publish what the mileage trigger is for these items on the Clarity, but they can be estimated based on service schedules for other Honda cars, and by user reports in here.
The prepaid maintenance plan is probably not a good idea for the Clarity if oil changes are every 12 months, and brake pads get very little use. If you put on a lot of miles, and drive in mountainous areas, then it could accelerate some of the wear and tear, but I still doubt that a service plan would be worthwhile. But I'd have to see the numbers to know for sure.
