Will the magnetic work if the phone has a case?
This wireless mag-safe power bank can worke very well with magsafe iphone 12 case, and if you use normal case, the magnet will be very weak , it do not support thick cases.

Is it normal that the battery gets hot while charging?
There may be heating caused by the principle of wireless charging when compared to conventional charging. These temperatures are well inside the normal operating temperatures of modern smartphones, please feel free to use it.