Is there anyway that INAV could open a window and at least show the live feed of the attached camera on the flight controller. It will simplify camera and VTX setup so easily. Currently to see whether your camera is actually sending out video feed is to use goggles or a screen with a fpv receiver. Very frustrating  and laborious  setup just to verify the camera,
I would like to see the live video while setting up the OSD. 
Alternately, is there any way that I could perhaps run the VIN (Video In from the camera) pin paralell to an arduino pinattached to a screen a screen. Thus I could at least see the video feed in real time?

Flight controllers don't actually input any digital video, so this is not possible. Instead, they just feed OSD information to the VTX and your digital receiver applies it as an overlay. If you have something that supports HDMI out, you could get a mini HDMI to full HDMI connector and attach it to a monitor or television.