[itsme]

A list of name / folder pairs for every device could solve that.
The names have to be the same on all devices, the folders are specific for each device.
Folders can have subfolders.

For example my usecase:

Acer-Tablet with Android 4.4:
MSP directory: /storage/emulated/0/_MobileSheets
Database: /storage/emulated/0/_MobileSheets/mobilesheets.db
Folder "Songs": /storage/emulated/0/_MobileSheets/Songs/
Folder "Fakebooks0": /storage/emulated/0/_MobileSheets/Fkbk0
Folder "Fakebooks1": /storage/sdcard1/Android/data/com.zubersoft.mobilesheetspro/files/Fkbk1/

Samsung S4 with Android 5.0.1:
MSP directory: /storage/emulated/0/_MobileSheets
Database: /storage/emulated/0/_MobileSheets/mobilesheets.db
Folder "Songs": /storage/emulated/0/_MobileSheets/Songs/
Folder "Fakebooks0": /storage/emulated/0/_MobileSheets/Fkbk0/
Folder "Fakebooks1": /storage/ExtSdCard/Android/data/com.zubersoft.mobilesheetspro/files/Fkbk1/

HP Detachable with Win10 1709 / Acer Notebook with Win10 1709:
MSP directory: d:\HerbertsGoogleDrive\_MobileSheets\
Database: c:\Users\her\AppData\Local\Packages\41730Zubersoft.MobileSheets_xxx\LocalState\mobilesheets.db
Folder "Songs": d:\HerbertsGoogleDrive\_MobileSheets\Songs\
Folder "Fakebooks0": d:\HerbertsGoogleDrive\_MobileSheets\Fkbk0\
Folder "Fakebooks1": d:\HerbertsGoogleDrive\_MobileSheets\Fkbk1\

I checked thoroughly on the detachable and on the tablet that MSP creates files, folders and subfolders as expected when I restore a backup from the same device and when I import files. These are currently the only cases when MSP needs write access to the song file folders. MSP on the S4 is just for a case of emergency, so it requires some more tests.
When I restore a backup from a different device (e.g. make a backup on the Acer tablet and restore it on the Samsung S4 or on a Win10 device) MSP works correctly, but some file locations are changed (files are moved into the root of the MSP folder). So automatic sync would completely confuse the folder structure.
With my folder structure I can put files that I want to edit with other programs into the local memory on Android ("Songs" and "Fakebooks0") and make full use of a lot of space on the external SD card for files that I don't expect to be edited locally ("Fakebooks1"). On Win10 I am able to edit every file, song files are synced between Win10 devices by GoogleDrive. Related files can be organized nicely in subfolders.

It's correct that write access is limited in different ways on the different OSs. I'm not free to choose any path on every device and it needs to be checked if MSP is able to create files and folders. Files in locations where MSP has no write access are probably not useful.
But I think the advantages of being able to use both internal memory and SD card on Android and sync between Android and Win10 are worth considering.
One issue is still open: the same path can show up under more than one name on Android. E.g. /storage/emulated/legacy/, /storage/sdcard0/ and /storage/emulated/0/ are different names for the same root folder in internal memory.