I apologize about the delay but Custom RBAC Support is now available in Production. You are able to to create role definitions at the MG scope and assign them to inherited MGs and Subs.
There is a bug that is in the portal where the new custom role is not showing when you are trying to do a role assignment on an inherited child MG/Sub. This should be resolved soon and PowerShell, CLI, and API are all working. I will not do any announcements yet on the availability of the feature until the portal bug is fixed. Once that is fixed we will do blog announcements and I will mark the feature complete here.
We continue evaluating several alternatives to provide full email customization. We are actively working on an alternative.
Unfortunately we do no yet have an ETA.
Still no news to share, just to add that we are investigating options on modifications for the App Service multi-tenant offering with enhanced capabilities.