Your post doesn't make much sense to me.
From memory, the trial version should be removed once you buy the full version (licence won't work with it). You then download the full version from your personal download centre (the link is in the activation Email that PixInsight sends you). It's been 2 years since I tried the trial, and I'm going off a not so great memory here, so I apologise in advance if my memory is wrong.
I've had zero issues with installing PixInsight on both a Mac and Computer, the latter only a few nights ago. Took less than a few minutes at most.
Modify/repair/remove is a Windows thing. Nothing to do with PixInsight.
Even though the application might be installed with the latest installer, there may still be updates. It would be impractical to keep having to update the installer for every single small patch update that is released. This is common with every single software application that runs on MS Windows that I can think of and has been that way since day dot.
With regards to the installer telling you licence is good etc, it's job is to install the application. Nothing more, and nothing less. Once the software is installed, there are options to check licence validation etc. That is again common with pretty much every MS Windows application since day dot.
It shouldn't take an hour to figure this all out. You are overthinking things, and expecting applications to work in a way that doesn't apply to software design in general.
Even the simplest of persons can make life hard for themselves.
