I run a 275 wide tyre on a ET22 wheel and i got my arches rolled before fitting the wheels, i think i would have got away with it anyway as i have around 10mm gap and the lip if i recall is around 10mm.
Your tyre and wheel being ET12 will stick out 10mm further than mine so to get the tyre back i think you would have to go with a 255 wide tyre (20 less than current as that would be 10mm either side) or get your arches rolled.
Hope that makes sense although hard to know for certain in the real world. Its not that big a job doing the rear arches, i got mine sanded and resprayed at same time as there was rust forming on the inner lips so killed 2 birds with one spray can. Cost around £300 and now i have peace of mind i can fit nearly any wheel.