Emergency alerts: is there any way to silence the "severe threat alerts" and "Amber Alerts" while I'm sleeping?
In the Emergency Alerts stock app -> Settings, there is an option "Emergency alert tone" with subtitle "Play a sample emergency alert tone."  When you do this, it plays the tone and a dialog says "The sound you hear now will be played when you receive an emergency alert.  The alert volume matches your phone's volume."  This indicates that you could avoid the alert sounding when you are asleep simply by silencing the phone when you go to sleep (or using an app to change the volume on a schedule).  

This doesn't discriminate between different classes of alerts (e.g. make sound for some but not others); if there's an app for that (probably is) someone else can post that as an answer.