Concurrent queries are supported (not sure what you mean by store being a single file, it is a directory of many files).
Concurrent indexing is also possible, so long as you can arrange to not put duplicate document ids into batches executing concurrently.
As for usage, it is just a library, so it is typically embedded in a single process (though this can serve multiple clients concurrently).
Distributing the index across multiple nodes is done at the application level. At Couchbase we do this with bleve in a separate project called 'cbft'.