Bethbryan All of those questions are answered with the GitHub link.
What I see there says:
Signal version: 7.16.4
Battery optimization: unrestricted
Stable internet connection
...which is not clearly an answer to the question from yore about battery optimization for the various parts of Play Services. If the part of Play that is waiting for the FCM alert to bridge it to Signal isn't running because it doesn't have permission to run, then Signal itself having permission to run isn't enough.
Note that when Play is running on Google's stock OS it doesn't need to have its battery usage adjusted, because it can do largely whatever it wants on the stock OS.