Catholics/Converting to Catholicism
Is there in your opinion any good reason for a Protestant to remain Protestant, after one's coming to believe all the teachings of the Catholic Church. For example, a person could maintain that remaining Protestant would allow an opportunity to work for change from within. Another example is that a person's nationality is a reason, for example, when a person is English and is a part of the Episcopal (Anglican) Church.
No, there is not. Only the "bad guys" have ever been known to be able to infiltrate an organization (of any kind), move in and take over, corrupting its original direction, replacing it with another direction. By continuing with a false organization or "church" all you succeed at doing is corrupting yourself.
As to the "English" example, one cannot be morally required to belong to any State's non-Catholic "State Religion" in order to be a good citizen of that State, indeed the truly best citizens (even though seldom to be treated well) are its Catholics. Only if citizenship is specifically and totally denied to Catholics must the person renounce any citizenship they would have in that traitor-State which is really at war against God. But there is no instance that a person should ever remain part of something false on some misguided excuse of somehow changing it from within, for the whole definition of the thing is to be false. To remain slyly within something false (being yourself "false" to that organization, making one a liar) on the off-chance of attaining some sufficient rank as to empower one to change its direction is quite a long shot indeed, and where does that place one in the meantime? Leave spying to deceivers; our stand is obvious and necessary.