If you are storing the status in a session variable then you can just reference that variable in your store order summary server behavior as the value for the status column. This will store whatever you have in the session variable into that column.
I'm not very clear on the scenario you are describing about the inventory id and the sizes, if you can please post back and explain this a little further, an explanation of how it should work and the desired result would be best. If you want to store an additional inventory id value in the order items table this should not be a problem if you have a column in the table to hold this value. Where this value comes from though will depend on your current setup and weather or not you have this value already available on your page. If the value is not available then you may need to query the db for it with a recordset.
