Hi, 
this is too complex to use a single multipolygon to contain all the landuse areas. I suggest using simple polygons instead and use multipolgons for areas where you have valid inners (I see you have a separate one like this) and where you want to share sides of adjoining polygons instead of duplicating ways. The area is already bound by the NP boundary
https://wiki.openstreetmap.org/wiki/Relation:multipolygon
https://wiki.openstreetmap.org/wiki/Multipolygon_Examples