I'm wondering that the addonmanager2901 might have some kind of bug(s).
The Addon Manager doesn't have any bugs, if it's not running in your system, it's because your Windows installation is missing some needed system libraries, or they are installed, but corrupted. Most likely, they are the MS Visual C++ libraries, which the Stand-Alone Manager installs anyway (using the official MS installer), but if they are corrupted, it can't fix them.
The GSX installer contains a *slightly* different version of the Addon Manager that requires a *slightly* different version of the MS VC++ libraries, which apparently work on your system.
For some users it's the opposite, the version that comes with GSX doesn't work, but the one in the Addon Manager installer does, because they have a different problem in *their* system.
The download messages are not really a problem, the issue is that the main download server has been overloaded with too many access due to the GSX release, so the installer got its files from a mirror site. The message is harmless, is saying that the main server couldn't be reached, but it was completed on the mirror.
the XPOI setting menu disappeared from the couatl-powered products menu
If you made a complete uninstall of the Addon Manager, that's to be expected (the uninstaller also warns about this), if you reinstall XPOI and then GSX again, everything should work.