Hey KindredCoda... Thanks for adding the GGB building structure! I'm wondering if a type=building relation would be better here. Then all the parts would be role=part. See https://wiki.openstreetmap.org/wiki/Relation:building
The way it is now as a multipolygon and all the parts that overlap each other as role=outer doesn't really work very well because "Generally, the multipolygon relation can be used to build multipolygons in compliance with the OGC Simple Feature standard (Graphical examples of OGC validity). Anything that is not a valid multipolygon according to this standard (e.g., polygons with intersecting rings) should also be considered an invalid multipolygon relation, with the notable exception of touching inner rings (see below)."

Just a thought - I don't think it would be hard to convert it - you've done all the hard work already.