No, the car will not move if you forget to unplug it. You will get a warning message on-screen. If you unplug and leave the flap open, the car will move, but it will warn you that it is open.
You don't need to charge to 80% to 'protect' the battery. BMW battery management is pretty good. Even when you charge to 100% you aren't charging to 100%, as the battery management software has set aside a percentage of the battery that can't be charged, as a buffer. One advantage of charging to 100% along with setting pre-conditioning, is as the battery approaches 100%, it will initiate battery cell balancing, which helps maintain overall battery health and range.
On the Facebook i3 world wide group the most common answer for your charging question is: "A B C", or 'always be charging". Either way works out the same. Eg. use 15% of the battery per day and charge every day, or use 45% of the battery over three days, and charge on the third day, is exactly the same thing as far as the battery is concerned.