The clients you have should also have access to the admin area if you want they to be able to manage theirs pages. Since version 3.6 we have implemented permissions per owner of pages or content items which means you can grant or deny a given owner independently of the other roles you have created.
The situation with "only one page" is more complex, but there are different approaches. For example you can disable creating of more pages once a user has created one page - it could be done manually or programmatically( in case you are developing some custom form for creating pages.)
the Telerik team