Does your Recordsource only return one line of data? If it does not than that is your Null error. The other issue is it could EMPTY which is not the same as NULL. You might consider adding...
Or strSite <> ""
Hmm, since each User should have a Front-end to work on not sure I see the problem? Are you saying you want to make updates the Front-end? If so then make the changes on the Master Front-end and redistribute to your Users.
In post #14 @arnelgp posted a sample you could build on. In post #20 I posted an example you could build on. You can also have a look at...
http://allenbrowne.com/AppInventory.html
Hmm, what is the Primary Key? Because that is what you should be filtering on. If you made the field that holds the Name the Primary Key then that's the problem. There needs to a separate one and you can use Autonumber as the Data Type.
So a Client gives you a Sales Order and you use that for the Purchase Order for the Supplier? How? You may want to order more than just the one Client ordered.
If you change the quantity on the Order than it also changes it for the Client. You also have to consider trying to to...