You intend to make a "plugin for a plugin" for Wordpress, where the plugin you're writing code to connect to is itself GPLv2, and you wish to know what your licensing obligations are.
The short answer is that GPLv2 s2b says that
You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License
There's no ambiguity or wiggle room here. If your program is a derivative of the existing wordpress plugin, you are bound by the original plugin's conditions for making derivative works, and so you must license the complete new work, when you distribute it, under GPLv2.
The question of "when is my code a derivative of another piece of code" is a matter of copyright law, not of licensing, and is harder to answer. For one thing it's jurisdictionally-dependent, and for another a number of jurisdictions lack significant case law on the subject. While courts are in no way obliged to respect it, the normal community line is to look to the FSF's interpretation, which frames the question in terms of the idea of the single combined work. It says that "It depends on how the main program invokes its plug-ins", boiling the analysis down to how your plugin and the GPL plugin interoperate. Many, including myself, have written more on that elsewhere on this site, and though you should read it we need not discuss it further here.
You write that "it will be depending on interpretation - that of judges", which is true, but not very helpful. The GPL has already been litigated a number of times, in a number of jurisdictions, as have some other copyleft licences, and they've stood up in court. If you're prosecuted for stealing a book from a shop, the outcome also depends on the interpretation of judges on the day. Historically, that interpretation has usually been that stealing is against the law, and you would be unwise to rely on it being different in your case. You also make the point that this will only apply if it goes to court, which is true, but are you sure you'd like to be the expensive test case?