I don't think disabling the submit button under the current setup would work, because many customers make basic errors (eg. forget to enter email address etc) and need to resubmit. This basic error checking is done my the same script ...

Disabling the submit button works entirely on the client side and has nothing to do with the server-side script - it really just prevents the submit button from being clicked twice in the browser. Also, re-enabling the submit button when necessary is just as easy as disabling it.