a) Normal / of no concern. LD matches the tubes electrically, glow brightness is not an indicator of balance or imbalance. The "burn mark" is called silvering, on the top of the tube. It's stuff they coat the inside with during manufacture. There's always variation.
b) Best practice is to turn off the amp when switching phones, waiting a few minutes, then proceeding. this might be overly cautious, but always do this when switching tubes, installing the jumpers, switching gain settings, or unpluging RCA's. Standard phono plug design is not very well conceived, but we're stuck with it. As the plug is removed or inserted, the circuit shorts +/- , which is pretty harsh on tubes and caps etc. Be kind to your new baby