Answering in English as German is not my Muttersprache:
I have seen more instances where huge areas had been made by making MPR with the outer made of several different other area parts and so on.
Maybe in a need to ‘save’ elements or what, but as you say: almost impossible to analyse, change or maintain; especially with other editors than JOSM.
If the (bad) practice is used to also include highway-parts as part of the area-landuse MPR, it becomes almost impossible to change anything.
If you can demonstrate that the area’s are indeed mapped in a wrong way, it might be better, after a deep breath, to remove the MPR and start mapping individual landuse areas from reliable high res photo’s.
The inner polygons you can keep as you take care that you split the former ‘outer’ areas around the inners or possible other landuse can be seen and used.
Keep your edits small and upload frequently to prevent conflicts and more.
And please: do not use highways for polygons.
