OK, I don't know what I did right this time, but after some 30 attempts that failed, THIS procedure worked:
1. Imported just one layer set
2. Opened up layers and
manually saved the current Custom set as new set
without clicking on the set imported above
3. Went back to Attribute Manager and imported another *single* layer set using Overwrite
4. Opened layers, and figured out that the second set is actually correct
5. Went back to Attribute Manager and successfully imported all other layer sets using Overwrite.
Here's a quote from email I got from our representative (thanks as always, Greg!), that explains why I kept failing with "Append":
If the file you brought across had layers which were not is exact numerical order for the in index values (for example, it had 1=Walls, 2=Floors, 3 was unused because it had been deleted, 4=Roofs) after "appending" the layers across you would not have the same index values (in the example you would end up with 1=Walls, 2=Floors and 3=Roofs). But a layer combination that originally showed the Walls and the Roofs but not the Floors (show layers with index values 1 and 4 only) would still try to show those index values (but since Roofs isn't 4 any more it would be incorrect).
Hope that maybe helps somebody.
