At 14:50, there are 2 lies active: "Gold in Red box" (red chest) and "The bottom row has a mimic." (blue chest). Therefore, the red box stating "There is no mimic among the red boxes" must be telling the truth, as there is no room for a lie to be told, allowing the middle red chest to be opened freely.

I think you got your left and right mixed up at 1:40 I'm not sure why you immediately decided "The Rightmost Column has a Mimic" was safe to open when you evidently hadn't realized that the top row had no mimics yet