Not convincing. If the "outer" of the forest is also the boundary of the nature reserve, then I don't see the reason why we should not used that as a "shared" border.

Also, I'm not sure what you meant by "very big multipolygon mess". Several multipolygons have hundreds, if not thousands of children. For example, this one https://www.openstreetmap.org/relation/9443832, and it was totally fine.