Sorry to hear about your trials. I am new to this forum, & have been travelling around US, Mexico & Canada for over a year now. We also had the intention of bringing our Diplomat home, to travel around Europe in it. When i checked ,the rules in NI were the same, since it is part of the UK & they are also ruled by Swansea. You also need to prove you live there before you can register a vehicle.
Have you tried Eire. i know a lot of people used to grey import Japanese cars through the South a few years ago because it was easier. I have also been thinking about registering in Spain.....but that could be a whole other can of worms.
When we left the UK 15 months ago this was all nice & simple. The reason we bought a Monaco was because we thought it was one of the few truly UK legal motorhomes!
Best laid plans etc.......