I believe the mounting points are different between the r and re. Make sure it fits!
It shouldn't do. It's the silencer that quiets things down. Worse case scenario you'll need to repal the silencer. Couple of quid for loft insulation should do the trick.
I MOT'd mine with the front frame mount not connected. No problems. DEP will pass an mot, i cant see why people might think they wouldnt? As far as the RE goes, im not sure how the emissions regulations work but i can see it ever causing a problem for an mot.
Nah mine went through no probs, if I'm not mistaken, if a pipe says not for road use, or a track only pipe on it they don't go through, but I know mates that put a sticker over it, that solves the problem there.