It wont be demonic influance he falls to, it'll be the sha. He hates anything to do with demons so more than likely wouldn't touch the stuff.
My theory from what we know is this.
Ji Firepaw becomes the hordes Ambassador he would likely tell the people in charge about his own people and there battles with the Sha. (This is probably what he later comes to regret) Garosh likes useing anything and everything as a weapon to be used against the alliance. He would more than likely put a group of people incharge of experimenting with sha energy/creatures. They would need somewhere out of the way to do this...
Ragefire Chasm
From the video we know that the dark shamans are a group of red skinned orcs. Garosh is extreamly paranoid about fel energy so probably wouldn't trust any green skined (fel tainted) orcs, and more than likely to get untainted maghar (red skined orcs) to work on it.
The Sha feed on anger/rage/hate. The element of Fire represents anger/rage/hate. So the people he would need to work on it would be shamans due to there closeness with the elements. But this would also mean that the shamans focused in useing fire, and would be suseptable to sha corruption.
Also at the end of the video (when Jessy turned in the Corrupted Amulets) the text said something along the lines of "this will show what happens when people fail" quite a large hint that they already knew about it and something went wrong, aka they became corrupted by Sha.
Considering how thick skulled Garosh is, I doubt something like this would be enough to make him realise that it wouldn't work, and carries on with the research. His closeness to the Sha energy and his constant state of simmering anger even when he isn't doing anything will be what causes his downfall in the end.