M5 doesn't come with sdcard.

Don't recall a phone that comes with a sdcard.

I did saw smart cover listed as one of the features in a flip case. But why would this be a problem? You only flip open the cover when you want to use the phone right? It shouldn't contribute to anymore battey drain because you would have unlocked it anyway?

As for battery I am currently doing some testing. It seems that with enough features turned off. The phone might really go 2 days. I managed to get more than 50% before I go to bed. With around 1.5 hours screen on time. Will report further here once I have finished my testing.

It shouldn't. It is just a magnet that should only wake up your phone when it is opened or closed. Loosing 40% at night is a bit bad. Few things that you can try to get better battery:
Turn off Google now
Turn off location history
Uninstall Google fit
Uninstall Facebook/messenger and use the web version
Disable account auto sync.

Check your battery usage under Power management. See which app is draining the battery. It takes just one misbehaving app to drain your battery.

It would be preferable if you monitor the battery the next day.
I can't think whether it's between user or phone's fault.

Tap the screen section. How many hours of screen time on?

Confused here. What's the battery % left now after 3.5 hours of screen on time?

I bought mine from Sony Center the curve. I sent my phone back there, but they just helped to send it to IXUN at Plaza Berjaya. So it is more convenient to send it to Sony Center coz it opens on weekends, just that it will take longer.

Not sure about Sony Authorized Service Center.

May I know which Sony Centre you bought from?

Centre Point got Sony Centre? I didn't know that. Anyhow, send to Ixsun @ Berjaya Plaza, opposite Berjaya Times Square. As most of the return repair Xperia mobile phones are being sent to that place.