I tested one of this case at a phone accessory shop before, it fits nicely.. Tho keep it mind that it is a thin plastic case so drop from a certain height may still damage ur phone..

I faced this once during early MIUI 8 revision i think, happened suddenly and the solution that works to me at that time was i just plugged it to my charger and let it charge without turning it on till battery is full.. when i tried turning on, it just works normally..
Not really sure what caused it tho, although when it happened i was using the RN3 till the battery was almost depleted..

I think it's normal, last time it happened to me also, check in settings but cannot see whats eating the space, had to use the app disk usage, then i know it was the stupid manga reader app that was eating my storage space.. delete the app storage and got back a few GB space and the warning went away..

I used the DiskUsage app by Ivan Volosyuk:

https://play.google.com/store/apps/details?...droid.diskusage

It will show all the data usage in ur storage, let it scan for a few minutes first..