(My bad, I had “building parts” layer hidden by default in iD.)
While I’m well-versed with multipolygons in general, I only have a limited experience with building parts (and I drew relatively simple ones). For reference, the documentation states that
The entire building outline should be filled with building:part=* areas, tagged with their respective height and other attributes. These areas may overlap each other or may be disjunct, depending on the building. That said, while 2D footprints can (and often need to) overlap, avoid overlapping 3D volumes – especially if the volumes have common faces.
For “pyramidal” shapes such as your rooftop, I would first try overlapping the inner parts onto outer, thus removing the need for multipolygons.
Someone recently posted a link to a test 3D renderer for buildings that you can toy with. I think it was @beakerboy (Showing off my 3D building renderer) – see e.g. https://beakerboy.github.io/OSMBuilding/index.html?id=1335900016 for your building. For the full list, see 3D - OpenStreetMap Wiki.
Even if you go for multipolygons, beware of caveats: 18312053 is a valid one, but 18312145 is not – it features bordering inner ways, and is therefore not a valid manifold.