Both of my GPU's work all the time, but Intel's load is 80-90% when playing games and Nvidia's is 20-30%. Whatever I do, I can't change that.
I did all the things you did in various options....
I wouldn't worry about what he said, looks like he only joined the site to go on about how good his own system is, as his post contains nothing of any help to you whatsoever.
Is it worth trying another brand of dummy plug? Or trying a HDMI extension cable with the dummy plug on the end? Reason I suggest this is it's cheap to try and the HDMI port on the laptop may need to sense some resistance from a cable to fully enable the dedicated GPU.