Here is my point of view for each of the questions. I tried to support them with a link to where the answer might come from. However, this is not an exact science so this might lead to discussion, which I think it's OK in order to get a better understanding on what can or can't be done.
1. No. If it is not licensed and most likely not even copyrighted then it would be part of the public domain.
2. The core is licensed under the GPL license. This means that you can license your add-ons only under GPL-Compatible licenses.
Source:
http://www.gnu.org/licenses/license-list.en.html#GPLCompatibleLicenses
3. Follow the steps in here:
http://www.gnu.org/licenses/gpl-howto.html
4. No. As the core is GPL it requires any piece of code that uses it to be GPL-Compatible. If your add-ons call the core then they should be GPL-Compatible. If your add-ons share data structures with the core they should be GPL-Compatible. If your add-ons do not work on their own and require the core to operate then they should be GPL-Compatible. Clearly, there is no way to run away from making your add-ons GPL-Compatible as they are considered derivatives of the core itself.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#GPLAndPlugins and
https://en.wikipedia.org/wiki/Proprietary_software
5. Yes, the GPL license allows you to do so.
Source:
http://www.gnu.org/philosophy/selling.en.html and
http://www.gnu.org/licenses/gpl-faq.en.html#DoesTheGPLAllowRequireFee
6. No, you don't have to. You can distribute it to anyone you want or not distribute it at all.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#GPLRequireSourcePostedPublic and
http://www.gnu.org/licenses/gpl-faq.en.html#UnreleasedMods
7. Yes. The GPL license allows them to distribute the add-ons to anyone and even charge for that.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#CompanyGPLCostsMoney
8. Definitely not. Thing is this also includes yourself. This means, the add-ons are by themselves derivatives of the Q2A core. So they have to keep the same copyright as the core. However, you can add yourself as a copyright owner of your add-on but you can not remove anyone from the original work.
9.1. No.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#DoesTheGPLAllowRequireFee
9.2. Yes.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#DoesTheGPLAllowRequireFee
9.3. It depends. If they have created a derivative work of your add-on, which is a derivative work of the core, then they can add themselves as copyright owners of the changes they have added. If they have not created any derivative work then they can not touch the copyright.
9.4. Yes, and they can distribute it and even even sell it.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#GPLCommercially
10. No. The GPL license does not allow you to do so.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#DoesTheGPLAllowRequireFee and
http://www.gnu.org/licenses/gpl-faq.en.html#CanDeveloperThirdParty
11. This question makes no sense at all. It is not a matter of something being appropriate/well-seen or not. It is a matter of violating the license or not. Distribution of an addon (which must be GPL-Compatible) under a GPL-Compatible license does not violate the license at all.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#CompanyGPLCostsMoney
12. As the add-on is a derivative of the Q2A core which is licensed under the GPL license then it must be licensed under a GPL-Compatible license. If it is licensed under a different license then it is in violation of the GPL license inherited by the Q2A core. Being strict you should report the violation. Being not so strict you could just remove the non-GPL-Compatible license, add the GPL-License and distribute it. Being not strict at all, you could contact the developer and ask them to correct this situation.
Source:
http://www.gnu.org/licenses/gpl-faq.en.html#GPLAndPlugins ,
http://www.gnu.org/licenses/gpl-faq.en.html#ReportingViolation and
http://www.gnu.org/licenses/gpl-faq.en.html#HeardOtherLicense
