Not exactly the answer to your questions but you may find this information helpful:
GrapheneOS will never include either Google Play services or another implementation of Google services like microG. It's possible to install Play services as a set of fully sandboxed apps without special privileges via our sandboxed Google Play compatibility layer.
and this answer
MicroG is not supported. You can install it if you want and it'll work to the extent that it can work as a regular app, but it will not receive any specific support or integration in the OS.
Maybe you also find some more information in this discussion:
https://discuss.grapheneos.org/d/4290-sandboxed-microg/18