Not certain in I understood correctly, but here's an attempt to answer question 3.
I haven't found/heard anything about quotas on the ftp server in the Asus router. The ftp daemon used is 'stupid-ftpd'. Source and more info is available at http://sourceforge.net/projects/stupid-ftpd/.
If you disable anonymous access and create a user 'anonymous' with password '*', this will work exacly like a real anonymous user, only you can set whatever access rights you want, including 'Read Only'. The new anonymous user will have access to the '/ftp_pub/' folder.
That's probably the closest you'll get to creating a shared read-only folder.
Royan