I give up. What is the purpose of "Map" on the inreach site, https://us0.inreach.garmin.com/Map?

Just what syncs where is a little bit complicated with the newer inReach devices. 

Data is divided into collections. Other than default "Library" (which is not exactly a collection), you create collections any way you want.

Each registered device has one so-called "active collection". Any data created on the device (for example, waypoints  marked on the device or tracks recorded on the device) is sent to the active collection. On the 66i, this is also true of the corresponding recorded "activity" when you use LiveTrack. 

In addition to the active collection, each device may have one or more "sync-d collections". As the name implies, data in sync-d collections is sent to the device when you sync.

You control the active collection and the set of sync-d collections for each device on the Plans & Devices tab at inreach.garmin.com. Look for the Collections button associated with each device.

If you add data manually (for example, if you mark a waypoint on the map at inreach.garmin.com, or if you plan a route in the Explore app), you specify the collection(s) to which to add it.

Everything is in the "Library". It's the default active collection, but you should not leave it that way. In order to sync successfully, each collection must be smaller than some unspecified maximum size. Over time, the Library will certainly exceed that size. Note that the "real copy" of every item is actually in the Library. Collections just contain references to things in the Library. Normally, this does not matter. But confusion can arise when hiding or deleting things. Just keep it in mind.

As far as the applications go: Although it does more, best to think of Garmin Express as handling firmware and map updates. This is also the best place to set up your home WiFi network on the device, and to register the device with the proper Garmin Connect account. Garmin Connect Mobile is the fundamental connectivity widget between your BlueTooth device and your phone. It is best to pair with your phone through Garmin Connect. Activities will also show up in GCM, but that's a secondary consideration for the 66i. The Explore app is the place to perform syncing between the inreach.garmin.com web site and the device. This sync includes contact information, message text, and pretty much everything except firmware updates. It's also easier to do visual route planning in the Explore app than on the web site. After pairing your phone and the 66i with GCM, you can add the device directly to the Explore app. Rather than browsing for compatible devices, just look for the specific device itself in the list in the Explore app.

It's important to use the SAME set of Garmin credentials for inreach.garmin.com (the credentials to which you registered the 66i during activation), Garmin Connect (both directly and if accessed from within Garmin Express), GCM, and the Explore app.

As an aside, BaseCamp is fairly useless for inReach devices.

The 66i is the first inReach device which is a full member of the Garmin ecosystem. This means that you do have a lot of other ways to manage waypoints and tracks, including BaseCamp (for as long as Garmin continues to support it). You can also just move .gpx files to and from the device via the file system if you're comfortable with that. (If you use them this way, you might want to set the device to record activities as both .fit and .gpx files, rather than the default of .fit only.)

In your situation, the main use of inreach.garmin.com will likely be its connection to MapShare. That allows family and friends to follow your track (as recorded via LiveTrack) in more or less real time. This is a feature that's only available through the web.

I understand your reluctance to haul the cell phone out in the back country. Probably no need to do so if preset and quick text messages meet your messaging needs. However, if you find yourself composing free-form messages in the field, you might find the Explore app useful for that. Even with partial word completion, the use of the virtual keyboard on the 66i itself is cumbersome. 

At home, the Explore app sync is still the easiest way to get contact information and preset/quick text message text onto the device. This is also how you get the more detailed track (the equivalent to the activity) onto the web site. Without the sync, all you get on the web site is the granular "sent" points, which get there via the Iridium network.

1) WHAT a post, should be stickied

2) Do you know if there is any appetite for Garmin to reduce the sync foot print? Express to Connect to Explore to Inreach online...and people give Suunto a hard time for their ecosystem

I think about the only thing that's likely to disappear in the short to medium term is the "wired sync" (using the old desktop sync program and a USB connection) between iR devices and the inReach/Explore web site. Back in the DeLorme days, that was the only sync channel. The rest of the creep is a result of attempts to integrate the iR line with the Garmin ecosystem and vice-versa. Remember, the Express/Connect/Explore combination supports a lot of Garmin devices other than iR.