#1. The Pope is preserved from error by the Holy Ghost whenever he speaks magisterially on matters of Faith and Morals. So such a situation would not arise.
#2. For the sake of argument, if he did proclaim a false, heretical doctrine he would immediately cease to be pope de jure as a heretic cannot be pope.
#3. Any Catholic who was aware that the pope's teaching was heretical yet accepted it would be guilty of the same heresy as the pope. Embracing an heretical doctrine simply because it was held by the pope would be no excuse. Of course, anyone who was unaware that doctrine was false could not be blamed for acccepting it.
What you are describing amounts to papolatry.
I don't know why you made that vague reference to Pius IX, if it is relevant point it out.