I understand your concerns about ensuring the integrity of your data when having to maintain the indexes yourself.
In some situations, using Cassandra's built in secondary indexes is more efficient -- when many rows contained the indexed value. Maybe your permissions fall in this category? Obviously, the advantage is that Cassandra will do the maintenance on the index for you.
For situations where secondary indexes are not recommended, you make your life a lot easier if all modifications of the indexed entity (like your user) is executed by one single piece of code, which is then also responsible for maintaining all associated indexes. And write tests to ensure that it works in all possible ways.
I understood that Cassandra 2.0 supports transactions. I haven't looked at it yet, but this could also help maintaining your data integrity, when a failed update of one of your indexes results in a rollback of the entire transaction.
I hope this is helpful to you.