Is it possible that in your 4.0 system the admin user just had a first/last name combination of "Unassigned"? I don't believe you could have an Account record with no owner - it had to be a user and was automatically assigned to the user who created the record. This was something not changeable. Your custom field - new _assignment could link back to a user but made to be nullable - the true Owner field is a system field and not really configurable. It is possible during your upgrade this admin account got mapped to a different user that didn't have the same first/last name.
1. So no - in 2011 you cannot make the owner field null - it must be assigned to a user or a team.
2. You could create a Team called "Unassigned" and assign all those records there, then if they require being assigned to a specific user you can do that as needed. All records need an owner.
3. I strongly recommend not attempting anything like this on the database. First off it is highly unsupported (as is any direct SQL modifications) and it would almost certainly break a great number of things. The application expects a value to be in that field, as such when queries are carried out it would attempt to join with other tables based on the value, if it is missing unexpectedly you would certainly see unpredictable results.