the peer-to-peer provider can also work in a hub-spoke topology similar to the server-client you have in mind. the only difference between the two types of providers is that the peer-to-peer provider can sync with any peer. whereas the offline providers
clients cant sync with one another.
you should be able to provision your sqlexpress for an upload only syncdirection to your sql server database.
as to the the schema change, Sync Fx doesnt automatically handle schema changes. if you have used the sqlsyncprovider, you will have to deprovision and reprovision the scope. not unless you're fine with modifying the sync fx database objects directly.