Only if it's disputed. If Bergoglio came out and said, "I know the Church teaches the Real Presence, but I don't believe in it." What declaration would be required? There's no uncertainty whatsoever. Only need for a declaration would be to establish definitively the mind of the Church regarding the matter in cases of dispute or doubt. And this would simply be a "clarification" regarding the mind of the Church. All that's essential is the mind of the Church, which does not require to be expressed in some formal docuмent. It is not possible for the Church to "depose", so the effect of any such declaration would be dubious it not entirely non-existent ... again, other than to clarify to everyone what the Church as a whole thinks when it's not inherently clear.
That's right.
From Who Shall Ascend?.......
"...However, even though the hierarchy cannot take legal action against an heretical pope, all of them together, or any one of them in particular, can condemn his teaching; they can accuse him before God's tribunal, warn him of his sins, and remind him of the divine wrath. Should this measure fail to produce any correction, they can denounce him before his subjects, the Catholic faithful, and warn them that they are not to listen to his teaching. Indeed, not only may the prelates of the Church do this, they have a most serious obligation to do it, an obligation which is as grave as the heresies are pernicious and scandalous. And if they fail to do this, they become a party to the pope's crimes, and will most certainly share in his punishment.
Moreover, where the bishops default in their solemn duty to protect the Church and God's Little Sheep, the priests and the
laypeople have not the right, but the duty, to raise their voices against an heretical pontiff. They not only raise their voices to God in prayer for the misguided man, but they also speak out to the bishops and the priests, and among themselves so as to warn their brothers and sisters in Christ that the plague of heresy has infected even their Holy Father, and has rendered him dangerous and unclean."