After going through those uninstalling steps, and rebooting - or not - do you find any Obsidian processes in your Task Manager?
I wonder whether there is actually some rogue process still running, or if it’s a glitch in the installer. I’m having trouble imagining how this could be happening. I wonder if there is somehow a hidden ghost duplicate installation somewhere on computer?
Or… is there anything else unique you can tell us about your setup? Did it just install to C:/ or did you install to some custom place, or network drive, or… anything else at all that you can think of that might be out of the ordinary?
Are you running any aggressive anti-virus/firewall software that might be causing side effects when installing? (Now I’m just making guesses up.)