Sharepoint Access Services: User priveleges with only macros (1 Viewer)

I have developed a database using access services on sharepoint 2013 to manage my company's transportation requests. Currently, I have the default web form as a Users form, which does an onLoad filter of the CurrentWebUser's email address, and then displays one of three navigation forms--admin, subordinate, and customer. Issue: A user can bypass the boiler plate Users form by navigating directly to a form in their web browser. So, a customer with the web address that points to my admin navigation form could simply type in the address and avoid the permissions check of the default page. Constraints: Data macros do not work. My sharepoint server administrators will not enable the sharepoint foundation feature on the server. Because data macros do not work, I am forced to use the macros available only in forms. I tried an onLoad for each navigation form to check user permissions: open the Users table, filter, grab permission level, pass back to the onLoad. However, I am unable to call browseTo from onLoad for each navigation page. Oddly enough, browseTo functions fine when called from the default Users page, but not after I do the openForm during the onLoad of each individual navigation form.