I know the video can be sent to the smartphone connected to the controller, but I was wondering if the video display can be sent to another device (tablet or even a monitor) or somehow mirrored to a second device? This would be beneficial in some case; example Search and Rescue missions.

Not without adding a 3rd party solution. I've seen people connect theirs to tablets that also have a seperate HDMI out then send that to goggles or a monitor. If you have an iOS device, the Lightning to HDMI module won't work because it doesn't appear to pass the data pins through...so its either usb or hdmi but not both.

I suppose you could try to send the screen via Airplay to an AppleTV for viewing, but I sure wouldn't want to pilot through that. Wifi interference might be an issue with that too.

i think it is possible with e.g. litchi app and inapp purchase litchi vue... but never tried it

I sure wish I could find a good solution to the OP's question as well.  The only current solution I know of is using a tablet such as the Nvidia Shield K1 and output to a pair of goggles that have an HDMI input.  I am unaware of a tablet that has an HDMI input that is descent quality and affordable.

Since I'm using the iPad mini 2 or my iphone I guess I'm holding out for the DJI goggles as my solution.