The value in a mainstream cooper is in it's originality. I'd remove the dash and go with a 3 clock cluster, source some proper cooper seats but if not the seats you have are often mistake for the correct ones and imo won't devalue the car.
I loved my mainstream and of the various mini's I've bought and sold over recent years, it's the one I'd wish I'd kept.
Re the seats
a couple question
would the frames be the same as a cooper?
if they were re covered could they be made the same as cooper seats
by the reading from Somerfords it looks a possibility
found original dashboard backing 3 clock cluster on the way
painted the original spot light brackets today.
Going for the original as possible option