Don't put any speakers behind the TV. It will literally block the sound, causing significant harm to your sound. Is it possible for you to angle the front left and right speakers to point to the listeners? I'm not sure how you would do this, but it would probably help if you angled them down. Another option would be to move just the front and right speakers down closer to ear level.
As for the sub placement, your room is indeed difficult. Is your fireplace flu typically open? I think the placement you show is probably ok. It may work better to put it in the corner where the computer is, except that you would then need to move the computer somewhere else. Either way, it doesn't hurt to experiment if you are so inclined.
I don't think you'll want to run both subs together because the VTF-3MK5 is a lot more powerful. It can be done by setting the VTF-2 to a lower level, but then it won't contribute much anyway. Perhaps consider putting the VTF-2 in another room