The point is that you're asking a lot here about complex things. It's not easy to answer that in a few lines and most of it doesn't even depend on NHibernate.
First thing: writing an occasionally connected Smart Client is hard. It's important to define some requirements first from a business perspective (for example, replication/synchronization strategies).
Quote:
The management wants to use a commercial O/R-mapper, but I would like to use nhibernate.
Could you motivate that a little bit more? Maybe the management has very good reasons for a commercial O/R mapper (for example, strong support for disconnected scenario's)?
I am probably playing the devil's advocate here, but it's important to realize that you shouldn't pick a product based on personal preferences, but for business reasons.