Hi,
This one, for example, uses aerials and PICC osm.org/relation/19284329 , this one uses the same lines as the previous one, with the addition of a simple line without "source:geometry" osm.org/relation/19284222

Yes, I see the problem with the school multipolygon. I need to finish a street in Saint-Nicolas (Liège) and then I'm going to map the road between Rochefort, Han-sur-Lesse, and Wellin based on aerials and PICC, in addition to adding 2-3 walls, hence this multipolygon in preparation for the import. Something I didn't mention in the comments, which is a mistake!

Tried the F thing on both id and JOSM and it didn't work. Can you tell me more about this?

However, this doesn't solve my problem; I don't see a way to create a simple polygon object designed with lines using different "source:geometry" values except by using a multi.

As for added complexity, I've given my reason for using this type of relationship in a very specific and rare case, and as soon as we get beyond that, I can agree with you. The multipolygon is a basic and common object that every beginner knows, and if not, it takes 10 seconds to understand how it works. As for modifying a multipolygon, whether in JOSM or id, I don't really see the problem, except for the reluctance to not do as usual which isn't complexity.