Your two scenarios are actually not much different from each other. you see N/A as the status for the local index on the partitioned table because there are no index segments at the global level -- the status is only meaningful at the partition level.

But you example of exchanging a partition when the table has only one index is a special case -- where you have multiple partitions and a global index you cannot preserve the indexing of the single partition when it is exchanged with a non-partitioned table because the global index contains references to all rows fof the partitioned table regardless of what partition they are in -- the rows are all "interleaved" in the same index structure.

So when you exchange a partition of a table with a global index you cannot expect the index information to be painlessly transferred with that data to the non-partitioned table.

Does that help?