Gentlemen, it is with great pleasure that I announce my alarm went off this morning and I arrived at work on time.
Unfortunately, with all the fiddling around, I'm not sure quite what I did to fix it. I think that maybe just uninstalling and reinstalling the clock app from Google play fixed the problem. Either that, or fully disabling my scheduled Do Not Disturb function.
EDIT: I spoke too soon. My alarms didn't go off this morning. BUT I think I know why now.
It turns out that there were two apps called Clock. One that was cloned from my old phone, one that came with the new phone:
All my alarms were set up in my old clock. But when I'm giving permissions and exemptions in the battery manager, I reckon thats affecting the new app, which has no alarms. So the old clock app is still getting randomly killed in the night.
I've uninstalled the old clock app and set up alarms in the new clock app.
I reckon that will fix the problem tomorrow, but I'll have to wait and see.