Do I need to keep the night mode on during the night?

Phone Assistant / Manager / Battery / Eco. ModeTurn off Eco Mode Phone Assistant / Manager / Battery / Optimized ModeDisable optimized mode […]

Device Settings / Battery / App Battery ManagementSelect Gentle Wakeup in the list and enable Allow all background activity Pin your app […]

No. You can even close the app and it will reappear on wakeup time.

Yes, you can configure it in the app settings “General/Clock Information”. You can configure this separately for the nightstand clock in settings […]