This is an interesting conversation as we really can never solve it, unless we interview all Protestants. I will concede the point to this extent to say that every single Protestant I have known, knows that their Church isn’t the Church Jesus founded and continues in it regardless of whether it is true or not.
But lets continue as this is entertaining to me…
I would never agree to the second statement the reverse never works, as no Catholic would join a Protestant group for truth but because of opinion, as no Protestant group I know of professes infallibility. They might go because of personal opinion but submitting to infallible truth is impossible for a Catholic to do and move to a Protestant belief system. There is either fact or opinion.
So if there is an absolute fact that is part of revealed truth such as Purgatory then it must be believed. To go to a rejection of it would be an acceptance of opinion which the Catholic can certainly do, and commonly does. But never going away to an infallible truth as that is part of Catholicism.
Regardless of your understanding of Catholicism if it is wrong then you are not submitting to truth, but to an erroneous opinion. See I could reject Catholicism because of the opinion that they are idol worshippers, which is me using my erroneous opinion to reject the truth that Catholics do not worship idols.
Which is why bad understandings or different opinions do not change truth but rather are just a rejection of God’s truth that is available to us if we just are open to God’s truth. To deny that would be to say that God is powerless to provide us with undenyable truth and He doesn’t have the power to let us know it with certainty.
In Christ
Scylla