“does the broker help in clustering or load balancing?”
Not any more so than what can be done with IS.
Keep in mind that broker territories do nothing for clustering and load-balancing. We can dive into this in another thread if desired.
“Can an integration server connect to more than one broker?”
As I understand it, no. This is serious and seemingly arbitrary constraint. It makes it very difficult for existing IS users to migrate from 4.6 and earlier since 4.6 and earlier had no such restriction.
“…how a document be published to a specific broker?”
For 6.0, this isn’t an issue since an IS can connect to only 1 broker. For pre-6.0, publishing is always done to one broker. To which broker is configured when setting up the service that performs the publish.