I let the task manager open for a while after closing to see, and 5min after Couatl is still runnning. I see on the forum, a few problems reported with that, and evrytime an answer for FSX and not P3D.
That's not normal, but explains your problem and confirm my suspicions.
Please note, It's not that Couatl must "do something" to close itself. What should happen, is that P3D (or FSX, it's the same), must send a message to close it and the act of closing is entirely outside our control. If it doesn't close, it can only be caused by some kind of problem in your system, possibly another installed module is causing a problem, so Couatl doesn't get the command to be closed.
We even added an additional safety in the Addon Manager than, when the sim closes, it will try to terminate Couatl.exe explicitly, but even this feature must assume the sim CAN send the close message, to the Addon Manager in this case.
In P3D when you install a module or any program, you don't have an alert like in FSX to start or not a module you know ?
No, there's no such feature in P3D, so it's entirely normal you won't see it.
So until now, i have to re-install GSX each time i start P3D, and it makes sometimes my other FSDT sceneries ( and i have quite a lot ) gone.
No, you don't. As I've said, it's enough to just TERMINATE Couatl.exe manually from the Task Manager, at least until you figure out what other installed addon is causing the sim not to be able to terminate its started executables.
I removed coatl from windows firewall ( created an entry and exit exceptions )
If the antivirus is a problem, you must add it to the ANTIVIRUS exception list (Windows Defender), not the firewall. But what we suggested, is adding to the antivirus exclusion list the whole P3D folder, which will speed up the cache regeneration time a lot.
turned the .Exe as admin
Not needed. Couatl is 100% UAC-compliant, so it won't ever try to access folders not belonging to you (the user), which means it runs perfectly with normal user permissions and doesn't need to run As Admin. We never suggested to do that.
and even tried "execute this program WIN 8 compatible".
The Compatibility mode should NOT be enabled. All our products are fully compatible with Windows 10, I use Windows 10, there's a thread in the forum about several used using Windows 10 and all of them confirmed they don't have any problem with it. We never suggested to enable any Compatibility mode. In fact, we always explicitly advised of not use it.
10mins now, from when i started to type this message.. Couatl still runing in task manager, using 28% of my CPU btw..
Couatl doesn't normally use any of your CPU (not 0.0%, but usually a figure you can't even measure on the task manager), when it's running normally, but it's possible this happened because when you exit the sim, it was still recreating the cache, but now is stuck waiting for the sim which is not running anymore, so that's possible it's using some CPU in this abnormal situation.
Also, i use custom voices sound for pushback ( the ones you can find on avsim, the robot voice of the push is a bit annoying, still wondering why you havn't customized yet ) and i need to reinstall them each time i reinstall GSX.
You don't have to reinstall GSX. See above.