On "Steve 1": Tastes vary. I would rather not modify a file when I want a single option to be different for a short time - I might forget to restore the file, and other things would misbehave. Your approach is fine for a configuration that is used exclusively for interactive operation only, but think what it could do to batch programs running unattended!
On "Steve 2": I explicitly stated Unix and named its 3 decades-old command processors, not any of their imitations - Linux, BSD, bash, zsh, etc. And the command "source" is only usable in some of them, not all! In ksh you can use the period (.) instead. Furthermore, IIRC, just rerunning the configuration file will not REMOVE entries which had already been set, and are not referenced in the current configuration file. For example, if you defined an alias, either from the command line or from another command procedure file (shell script in *nix, batch file in PC) it will remain. AFAIK you cannot restore all defaults! As to TCC's two-file approach, different syntax is used in .INI files to overcome the limitations of batch file syntax - it would require very torturous methods to set some of the directives in commands.
For your questions directed to Rex, there are many directives that can only be processed when a new instance of TCC starts, from the .INI file; modifying of the .INI file by any technique, including the OPTION dialog, will take effect only when a new instance is started. You can continue using the old values in the TCC instance which was used to modify them.