The base setting for the low speed mixture needle is usually flush with the end of the throttle valve (usually about 1/4-1/2 turn leaner from flush with the outside of the throttle arm. As for the clutch shoes/springs - if the springs are shiny at all (it appears yours are black) or if they are blued at all, the shoes are too worn and that would cause the springs to lose some tension and engage the clutch too soon. If a clutch spring is broken, it could cause one shoe to engage at idle and stopping the car would put enough drag on the engine to stop it. If the springs aren't broken and no showing wear, then I'd be looking at the throttle linkage and the needle settings to ensure the throttle valve isn't closing too much under braking and the mixture isn't too lean.

Closing the idle gap will lean your idle mixture, so perhaps setting your mixture needles a fair bit fatter and "starting over" may be in order of the clutch is indeed up to snuff.