On my car the rear calipers are mounted on the back of the upright, therefore my handbrake cables head out backwards. I don't have an issue with this arrangement as I know some other cars do the same.

The cables from the MX5 donor are fairly stiff, so at the moment they loop back with quite a large radius. I'd like to pull them in a little to reduce the curve, but how tight is too tight? Is there a way of determining how far I can bend them before I compromise their function?

Too tight a radius increases the friction between inner and outer and they wont return properly. It'll also cause them to wear quicker but thats unlikely to be an issue on a kit. I'd guess at 3" radius at most (but just a guess).