Okay, so here's what I have back from Proton on this issue.
On ProtonDrive:
Regarding the clarification on AlarmManager usage and why we don't think it is a concern privacy-wise, we should separate AlarmManager itself (used as part of androidx.work libs visible in the screenshot), which is the core Android component and part of AOSP, and the com.google...AlarmManagerBroadcastReceiver , which is instead a tool from Google, which is part of the Firebase library.
Regarding the latter, it's not open source, therefore without trusting Google, it could indeed be a privacy concern.
(Note: the screenshot mentioned in the above reply refers to the list quoted in my post #1 in this thread.)
And ProtonCalendar:
As for the Calendar app, we generate event notifications locally and not via Firebase. AlarmManager is used, however it is the trustworthy version AOSP android.app.AlarmManager and not the com.google...AlarmManagerBroadcast which is part of Firebase lib, therefore the Calendar app is not affected by this.
They confirmed what was already discussed in this thread about the Protonmail app attempts to generate notifications using Firebase even when notifications are turned off and Play Services is not installed.
Anyone able to comment on this? I can do without the Mail app but Drive and Calendar would be useful but I wish I understood better. Exodus shows no trackers for these apps. Am I concerned over nothing? I don't have an extreme threat model but I'd like to fully degoogle on my GOS devices if possible.