Keesje,
First of all, congratulations on the very well thought out and informative concept. I must say the renderings & especially the video look absolutely fantastic and you've created a magnificent looking aircraft.
Just a little comment or two...
I would drop the thrusting APU concept. It's a good, and logical idea, but 30,000lbs+ of thrust would be a very tall order.
A simplified afterburner / reheat system on an existing turbofan such as a GE90 would be an easier option because by it's very nature it would be mechanically simpler, and a nice little bonus would be that during it's operation, it would burn great amount of fuel, which in turn would help get the aircraft down to landing weight much quicker.
I think the ingredients should be 2 x Genx'd GE90-115 with reheat. The APU could be a fuel cell.