1. To Navigate between ascx files - you will need to register both controls in the Moduel Definitions module on the Host menu.  The default control you create with no "Key" the other control(s) must have a Unique Key.  You then create the link using the various overloads of EditURL or NavigateURL

2. You can use any architecture you want - however you will find it easier to use the core acrhitecture as most of the grunt work is done for you.

3. Sure create multiple ascx files - register them all as in (1).  Then in the menu you will need to make sure that the correct url is generated to load the correct ascx file.

See the Module Development Guide in the Documentation package for more infor.