Great write up and good pics.
As Lawsy says the back of the seats (indeed the centre rear seat section as well) are prone to marking very easily. I'm not sure if a clear or coloured wrap will work as I'm not sure what will be the result when it is removed, but might be worth it.
As for the rear joint to the exhaust, they all rust. You might be able to pick one up but M3 ones are slightly different weight metal so watch out. Best way is to get it off and get an exhaust firm to make up a bracket out f standard parts OR get replacement rings from BMW and get them cut and welded on the box and centres section.
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