The pineal gland that real world and mirror world Ivan Avi don't have that allows them to communicate across the parallel worlds is actually a real gland. It sits at the base of the back of your head, right between the two halves of your brain and is the size of a grain of rice.
The Mirror Middleman looks and talks like Snake Plissken from Escape From New York, the villain's house address is 199 Plissken Ave.
The guns used by the alternate Middleman and the alternate Wendy are the FN Herstal model F2000, without the optional optical sight.
Although the Middleman's Theory that the perpetrator was related to the argument between Edison and Tesla as to who invented the light bulb is amusing, in reality neither one did. The original light bulb inventor isn't known but Edison did produce the first bulb that lasted long enough to make them a commercially viable product.
Despite Wendy's comment, the hat shown on the 4th etched mirror is actually the shape of a Panama hat. To be one, the only requirement is that it's made of straw, which you can't tell by the picture.