[ghmerrill]

If you can select Import->Local File and import files off the device such as the Downloads folder (which is typically located under /storage/emulated/0/Downloads on a regular Android device), then the all files access permission is granted properly (which also grants the storage permission). If you find that you can't access that location, then that permission is not granted.

There seems to be something of an anomaly here. When I select Import->Local File I'm shown a list of what are apparently the local files in my (current) song list.  The path at the top of that display indicates that they are in "root . storage . emulated ... files".  Fair enough.

There is a bright orange SD card icon in the page banner area.  When I click on that I get the "Grant Storage Access" image displaying 4 "Steps".  I don't really know what to make of that, but I have two choices: Cancel or OK.  Clicking OK displays a page titled "My files" with a "Can't use this folder" warning area.  I'm not sure what "this folder" refers to, but I'm assuming it refers to the "My files" folder, and it offers me the opportunity to "CREATE NEW FOLDER".  This is how I created my MobileSheetsStorage folder within the "My Files" folder.

Below that "Can't use this folder" box is a list of folders:  .Trash, Camera, Downloads, MobileSheetsStorage, and MyDocuments.  I have no idea why these are displayed.  However I can double-click on them and see their contents -- except for the case of MyDocuments, which brings up a"Select Files" page listing some copies of my current song collection (but not actually the ones in it at the moment).  I don't know what to make of that.  This is NOT what is displayed when I look at My Documents from the Chromebook Files browser, and it appears to have thrown me back into navigation within MobileSheets at the primary Recent/Pieces/Setlists/Collections/Compositions/Years page. The interface seems to be behaving somewhat oddly there.

In any event, given the criteria your propose, it appears that indeed the appropriate permission has been granted.

My speculation at this point is that MobileSheets is making certain assumptions about the Chromebook/Trogdor file system's similarity to the standard Android phone file system, and that some of these assumptions aren't valid.  But that's just a guess.  But I don't think I want to push the envelope any further in this direction.

I loaded up my Google Pixel Slate and for me, I have to do the following to get to that setting:
Settings->Apps->Manage Google Play preferences->Android Settings->Apps->Special App Access->All files access->MobileSheets
and then I can toggle it on. Please check that for me on your device.

Doing that check, "All files access"for Mobile sheet is shown as "Allowed".

As far as the "My Files", I would strongly advise against trying to make that the storage location. MobileSheets cannot get writable access to anything under that folder. I believe you should get an error if you try to do that, as MobileSheets will be unable to write to the folder you selected. 

I was allowed to create the directory, and there were no errors displayed either at creation time or subsequently -- at least not that I noticed. It simply remains empty. I don't know at what point and in what form the error would appear.  There was no error dialog displayed or anything announced as an "error".

The interface at that point is a bit arcane, and I recall at certain points something about certain "locations" not being usable but it wasn't clear what the reference was at that point.  I assumed that if something wasn't usable, then it wouldn't be created.  So maybe what happened is that I managed to create a location unusable by MobileSheets even though that location has all the necessary permissions.

It's natural for a user to suppose that his local storage would be a good candidate for such storage rather than other more esoteric (but also local) locations that are not visible to him in any immediate way.  And the Settings UI makes no mention of avoiding My Files (unless that was what was intended in the vague statement about a location not being usable.

I guess when I select the Import > Local Files > My files > Grant Storage Access path and see the "Can't use the folder" warning, I should just quit and realize that I can't succeed.

In any event, I'm going to abandon my experimentation with this since it's not a major issue to me, and I'll just let MobileSheets put stuff where it wants.

... I'm not sure exactly what your goal is with this. You can also create a library backup under Settings->Backup and Restore->Backup Library which contains everything in your entire library and all application settings if you are just wanting a backup.

My goal was to create a restorable backup of a collection (not just the files in it) -- largely to facilitate experimentation and learning with MobileSheets without having to repeatedly reload files and re-add metadata.  I did see how to do that and have those backups now.