I may have missed an important post, as - I confess - I just skimmed the Thread: The M2 - in common with most modern cars - has a monocoque chassis in which all parts of the body structure (and some bolt on braces) form part of the chassis. So, if this replaces just the roof skin, it's likely to be cosmetic. If it replaces the braces between the roof pillars as well it may still be cosmetic, but is also structural.
In the meantime, here's a couple of links to help the argument along.
https://www.realoem.com/bmw/enUS/sho...diagId=41_2163
https://www.realoem.com/bmw/enUS/sho...diagId=03_0183