Well I’m extremely disappointed about there being no response to this as I have the exact same issue! I just bought GSX this evening, and been through the same stuff as you.
I first tried adding the addon folder to Defenders exclusion list, that did nothing.
I then tried the offline addon manager install, which did seem to install into the correct places, as before in my appdata/addon manager/coualt64 there was nothing, now there are files and stuff in there.
I went into the installer, but it’s still showing as though GSX isn’t installed, and clicking the install button doesn’t do anything.
I don’t know what to do either?