In terms of a monitor, the Ninja V is great (I personally own the Shogun and Ninja Assassins). You will find working with prores in post is fantastic and you will love the extra bitrate. Also, because you are detached from your camera once the exposure and composition is fixed you can do all your recording with the Ninja by simply touching the screen. Saves you climbing up a ladder every time you want to press record!
Extra-long cables are pretty easy to find on sites like B&H. Not all HDMI cables are the same though so check the speed requirements of the ninja for cabling. I wouldn’t recommend daisy-chaining HDMI’s.
I haven’t tried Morgans idea about a mirror but it sounds like an obvious and easy fix if you can make it work.