Hi Christian,
I'm familiar with NI so I know the score about the antics that go on there!
To be clear though, I'm pretty sure CRA2015 doesn't exist there in the same format, same in Scotland. I don't know what does, but that could work in your favour.
Was the car MOT'd in NI prior to sale? If so, that means it was done by vosa as they are government run test centres, with one usually in each major town/city. Not like over here where there are a choice of 10 in every village. Ok, no pdi, bit the above should prove it had a pretty thorough going over prior to going to buyer.
Perhaps I've missed it, but where did you sell the car to? Local buyer or over here, mainland UK?
I wish you the best with this, but best to tell us everything you can about the situation so that when we are offering advice it is relevant as 5 pages in your info is coming in stop start.
Cheers.
P. S - please tell me this wasn't a Southern Irish imported car that you sold?