Running the same OS and WC versions as you, on a Mac mini. My settings are similar to
Randall's except I don't even bother sleeping the drives. I do require a password on all my machines, mainly because they are running as Admin. However, a power outage doesn't prevent the machine from Starting up and restarting WC, I just have to log in to check on things. BTW, I don't even have a monitor on the mini, but I still have Display sleep at the first mark before "15 minutes"/
There is one thing you didn't mention in your Energy Saver settings: "Power Nap". Be sure that is
not enabled.
Your Auto Login and Screen Saver settings not 'sticking' sounds like a System Prefs corruption. Do you have any Utility apps?
Are you using an Uninterruptible Power Supply? If so, is it set to shut down the Mac?
I tend to agree that WU has problems, however, if your Mac shuts down for long enough, WU will send these types of messages to you. I'd suggest looking at Console for any kind of "shutdown" and/or "Restart" messages. Especially any that occur on the same day.