Dump all balloons and all part lists then Inventor will allow structure choice.
The first balloon or part list sets the structure for all balloons and part lists in that drawing.
Also you can't expand the assemblies because the part list is set to first level, it needs to be set to all levels to allow expanded view.
Just as you posted I think I found a solution without deleting part list and balloons.
1. open model assy set BOM editor set structured view to all levels.
2. create new drawing.
3. insert a view of the assy
4. place part list structured, all levels, and edit to expand a few sub assys.
5. copy all sheets from original drawing to new drawing.
7. check copied sheets part lists to see if they allow the structure to be expanded.
8. If all is correct delete first sheet and save then check if part list structure still holds, when you insert a new part list it says first level but comes in allowing assys to expand, this maybe a problem, I don't know.
I have tried this and it seems to work.
As you have deleted all pl & balloons then it's easier to insert new pl.
Go to model BOM and set structured view to all levels.
Make sure the model BOM is set to structured all levels.
Then try making a new clean drawing, drop in the assy and place pl to see if all ok.
If that works then there is still a balloon or pl in the original drawing.