This is definitely not ethical. When I choose GPL instead of, say BSD, my intention is clear - if you benefit from my code, then you SHOULD contribute back. And it is very clear how I expect you to contribute back - by providing full access to the COMPLETE system which is using my code. At the core of GPL, the intention is that someone else should be able to make modifications to such a system and build other things from it.
With steps 3.1/3.2 there are sociological problems. Who will the user ask when his system stops working? Especially when the problem is with the GPL plugin, will the GPL plugin author support such a user? Will the GPL plugin developer entertain the unethical closed-source application developer?
Given the above, nobody will attempt your question 4, 5 & 6 in a large enough scale to get noticed. Also, if you are writing commercial application from which you make lot of money, then you can typically license that GPL code from the copyright holder for use in your project for some price.
To answer question 3, if you are the sole author of GPL code, then you have full copyright to that code. You can do whatever you want with it, including using it in non-GPL project even while you have released it under GPL for others to use.