1) essentially correct, but if you have a script that has the "run with full access" option enabled, this option enables full access interaction with the current file but not another file such as the data file. In some cases, a script defined to run with full access privileges must be defined in the data file and performed from a script in the interface file in order to run with proper access privileges.
2) No. With table occurrences defined in the interface file, the variables and most scripts all are part of the same file. Global variables are an option for any scripts that must execute in the data file, but values can also be passed as script parameters.
3, Q1) no, but it's easy to set up one copy of your system for one client and a different copy of the files for another.
3, Q2) Some relationships must be defined in the data file if there are calculation fields that require the relationship in order to evaluate correctly. Most if not all relationships must be defined in the interface file or the scripts, layouts, value lists, etc. will not funciton correctly. You'll need to analyze how each relationship is used to determine whether it is required or not in the two files.
3, Q3) Most files designed for use in FileMaker Pro as a single user file can be used with little or no changes when hosted by FileMaker Pro or FileMaker Server. With all such files, there are some issues that have to be checked, in particular the location of any "by reference" inserted files may need to have their references updated after the move and global fields function a bit differently in multi-user systems. Data Separation files have the same potential issues but nothing in addition IF both files are hosted from the server. If you install separate copies of the data separation file on each client machine, then the external data source references in the interface file will need updating in order to find and access the hosted data file.