Any question answered...

Another thing is that the centre bolt I have seems to be two different widths in different places - one for the lock ring type nut that holds the spring in place, and a slightly narrower one for the nut that goes behind the brake bridge.

Is this standard and will the replacement ones be the same? Where even are the replacement bolts, I couldn't find them on SJS.

Standard size is M7 for the caliper and lock nut and M6 for the retaining nut. I'm assuming your "Miche" brakes are actually made by Tektro anyway, they look like the kind of thing Tektro would come up with if asked for a set of brakes for another manufacturer.