You make a good point here in that Canon 188.4 of the 1917 Code of Canon Law states that a cleric that publicly defects from the Catholic Faith loses his office, by tacit resignation, by the very fact of his public defection. Therefore, in this sense one can canonically judge that a putative pope is actually not pope.
Firstly, those of the conciliar church would all say they haven't defected from the Faith; they are still catholic. They would say they are modernizing the Church but that such is allowed. So, there would have to be a case (similar to Martin Luther) to prove guilt.
Secondly, in any just legal system, one is innocent until proven guilty. You, however, are assuming guilt with no due process (even if there is some evidence). This is not only uncatholic, but unjust and you're acting no better than the famous tyrants of history. You've made yourself judge, jury and executioner.

It reminds me of the famous scene in the St Thomas More movie, "A Man for All Seasons", where St Thomas' son-in-law is arguing about the legal process with St Thomas.
“William Roper: “So, now you give the Devil the benefit of law!”Sir Thomas More: “Yes! What would you do? Cut a great road through the law to get after the Devil?”William Roper: “Yes, I'd cut down every law in England to do that!”Sir Thomas More: “Oh? And when the last law was down, and the Devil turned 'round on you, where would you hide, Roper, the laws all being flat? This country is planted thick with laws, from coast to coast, Man's laws, not God's! And if you cut them down, and you're just the man to do it, do you really think you could stand upright in the winds that would blow then? Yes, I'd give the Devil benefit of law, for my own safety's sake!”