It all comes down to interpretation. If you're of the view that Farah kicked the ball out of Inglis' hands into him, and if you interpret Inglis as not playing at the ball at that moment. Then its a try.
I don't like the above interpretation because it assumes Farah was kicking at the ball, rather than just trying to place his foot between the ball and the ingoal. I also don't like it because it puts no responsibility on Inglis' part in moving the ball into Farah's foot in the first place. It was his motion of planting the ball down that caused the ball to hit Farah's foot and whilst still in the same motion, it came forward off his arm.
Just need more clarification in the rules I suppose. I would like to what the interpretation will be next time when someone gets up to play the ball, hits the opponents foot in trying to play it and the ball comes forward. Pretty sure thats been ruled a knock on in majority of cases.
I think the fundamental difference in interpretation is whether Farah played at the ball or not. The unnatural movement of his foot tells me he did, and as such, your first paragraph applies and it is a fair try.
In fairness, common sense says it shouldn't have been a try, but that is because of the exagerated amount of rules and guidelines, one of them being the BOTD, which I think is total BS, because it allows players to claim a try when they clearly had lost control of the ball.
IMO the video ref should be used to look at possible infringements leading up to a try, and categoric proof the ball was grounded properly. He can ask for all the possible angles in search of that proof, but FMD, don't call a try because there is a chance a hair from a players forearm is touching the ball as it touches the turf.