We are trying to move away from Clasic Enrolment (where we have to side-load all the application via Profile / Package) to a more robust deployment (via Enterprise Google Play Store). 

I've managed to create the binding between the MDM and Google Play Store and get any application available in Google Play Store to get installed on the Work Profile (we are using COPE- Corporate - Personal Enrolment method). I was also able to upload a custom Android application via the same method. The issue surfaced when I've try to upgrade the custom application to a new version. When attempt to edit the custom application and load the new apk, I am getting Cannot update a published APK.