Oh dear! Good luck with the BIVA. On a positive note, I'm glad to hear from an expert that cutting away most of the rear pillars and the rear section of the roof frame doesn't compromise the integrity of the base unit in any way.
It would have to have been done 1988 or earlier, and you would have to prove it was done then, which isn't easy if it wasn't a conversion that was registered with DVLA at the time. Why would anyone bother?
I think the dodgy roof treatment is just a distraction from the other problems - what about the fact that it appears to have a 2000 SC engine in what should be a 2200 TC, the proliferation of tacky pretend wood stuck all over the interior, and the extra zero incorrectly added to the end of the price tag!