I assume that they are both hub-centric, so the only reason why they couldn't be crossed over (wheel offsets and rubbing aside) would be that the center hub were different sizes between the two.
Both the Mini Cooper and the BMW i3, have the same hub size. The hub center bore is 66.6 mm for both.
As you noted, the difference comes in the offset. the i3 wheel has a positive offset of 43 mm front and 53 mm rear , and 33/39 mm for the i3S - and size of wheel.
The Mini Cooper shows an offset of anywhere from 46 mm to 54 mm depending on the wheel size ( 14" to 18")
Not sure I'd want to try the smaller Mini Cooper wheel on a car that OEM's a 19 though, you'd get that 'Ricer' look, and you'd be dragging the battery pack on the pavement every time you hit a bump.
This site will allow you to see and compare specs.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 19 inch 427 wheels, EVoInnovate EVSE