A scripted plugin is simply a script disguised or run by a plugin. What are the reasons that you would like to write a scripted plugin rather than a plain script? The answer to this question will help us lead you in the right direction.
Initially, what Fred says: a "Scripting plugin" is a script, which is quite something different than a *real* plugin. Writing real plugins is so difficult, it has its own forum: InDesign SDK.
If you intend to write a real plugin, then most of your questions are better asked over there.
"Real" plugins can read and change literally everything in the entire program, but must be programmed in C++ and compiled separately into a plugin for Windows and Mac.
@Fred: I'm interested in writing scripts with ExtendScript language, and I plan to use them to enrich Indesing / incopy / idserver. The execution via script panel is ok.
OK then. With respect to your numbered questinos:
2. Scripting plugins don't need a debug version
3. There are no prefix or script IDs for scripts.
In that case you don't need the debug version nor a prefix ID. Becoming a partner is also not necessary. All you need is InDesign and Extendscript (which is installed with InDesign). this forum is full of experts on scripting that can help you with a specific problem. From Extendscript you will find all the documentation necessary to get you on your way under the Help menu. There is a object model viewer and PDFs that have tons of info.
Thanks to all! That's a relief!