Doesn't matter, 3 of the 4 justices held that a state could unilaterally secede.
Yes it does. Assuming, for the sake of arguement, that you are correct courts can still only rule on actions which have actually happened. Since no state had actually seceded, the any comments made on secession in the Penhallow decision were made in dicta and under the doctrine of stare decisis are therefore not binding.