indexes are together with the data. the partition key becomes the _routing key, so you can always search 1, x, or all nodes depending on your _routing value
lightweight transactions are only used on schema-changes (which are/should-be rare)
lightweight transactions are only used on schema-changes (which are/should-be rare)
As for the indexes - are they standard Cassandra secondary indexes? "Custom secondary indexes" - does that mean that it just looks like a secondary index, but is actually backed by Elastic search?
Though you can't query it from cassandra yet. You have to use the elastic-search rest-api.