So I suppose we can use the new full installer to install on top of our existing PHNL installation.
Yes, uninstalling or not first, will not make any difference.
And our registration-activation of the scenery will remain untouched.
The activation is never linked to the install process. You can uninstall/reinstall a scenery an unlimited number of times, and even remove Flight sim, it won't affect the activation, which is stored in Windows so, the only time a reactivation is needed, is when changing hardware or reinstalling Windows from scratch.
As will our assigned AES credits (yes, I know AES is not your responsibility).
No, it's not, but I don't see how any of the fixes could possibly affect it, except the fact that all our sceneries comes with GSX support by default, so the usual care when using them together is needed.