Can anyone suggest a way to mount my rack to prevent it impeding the rear brake?
You shouldn't even need to heat up the arms.  Just bend them whichever way you'd like; it's easiest if you have access to a vice.  You might even look at having them make an 'X', if that pulls the mounting arm away from the brake.   By design, the horizontal stays should have a minimum amount of force on them - they handle keeping the rack from rotating forward and aft while the bulk of the weight is transmitted down the vertical stays.

You could also get p-clamps and attach the rack stays lower or higher as needed to clear the brake.