Waypoints and routes created on the Mac (Garmin Explore) don't sync to the InReach Mini.

You may be tangled up in the whole "collections" thing. 

There's no good way to explain this briefly. Your data is/can be organized into collections.

By default, there is only the "Library", which looks like a collection but isn't quite the same thing. ALL of your data ends up in the Library. An actual collection represents a subset of what's in the library. The data isn't actually stored in the collection - what is stored is a set of references to data in the library. Still, from the outside, it looks like the collection contains the data.

Each device can have one or more associated collections. Each device can (and should ALWAYS) have a single "active" collection. Anything generated on the device (for example, a waypoint marked on the device) is sync-d to the active collection (as well as the library, of course). And anything placed in the active collection on the web site is sync-d to the device.

Note that you can change the active collection at any time. But you can only have one active at a time.

You can optionally have one or more "sync-d" collections. Data added to these collections on the web site is sync-d to the device.

You manage collections from a combination of the Map tab on the explore.garmin.com web site (general collection and library management) and the Plans and Devices tab (assign the active collection and any additional sync-d collections to a device).

Thanks for your input and useful explanation.  I'd stumbled onto collections after I'd posted my question, mainly because I'd run out of other things to try.  Once I'd found the key the rest came pretty simply, but I wish I could have had the information you provided a bit earlier and saved myself a few hours of groping for a solution.  Again thanks.