We have worked out a process to add and link customers and contancts. We foudn the LEadscontacts together messy as errors were confusing as the process was doing several things with only one set of errors in the error file so we do:

Create a unique name for each contact to avoid duplicates (add the date)
Add contacts leaving company field Null
Add Leads linking to primary contact

This seems to work fine except:

We allocate some of the Leads to a Partner. When logged in as the partner a Search List view shows all Leads and primary contacts in a nice view. Opening a record however there are no contacts attached. Also the contact view shows no contacts.