So a few hours later, I found the answer to my problem in the Filemaker documenation (http://www.filemaker.com/11help/html/passwords.13.2.html), the very last paragraph, which says in part...
Note If you have a multi-file database solution that includes multiple protected files, you may want to consider using identical account names and passwords in each protected file. When one protected file attempts to access another protected file (such as to access related data or execute a in the second file), FileMaker Pro initially attempts to open the second file with the same account name and password that was used to open the first file. If there is a matching account name and password, FileMaker Pro skips displaying the account/password dialog box. If there is no matching account, then FileMaker Pro displays the account/password dialog box so the user can enter account information.
the line above that begins with FileMaker Pro Initially attempts...was the key. My local app was starting up in Guest access and then I'd request user to login to a registered account. By merely switching to having the app launch logging in via the Admin account and then re-login to the guest account during the startup process resolved the access privileges errors I was getting.
It is curious to note that even though in the original solution I was later logging in as a user with full privileges, because the session's initial log in was through guest privileges, all subsequent access to the remote data was based on the initial login credentials of the guest user.
I hope this helps others avoid the hours I spent noodling this problem out. Or, if you have further insight that I missing, please post.