Papal Infallibility Raises more Questions than it Answers

Suppose papal infallibility is true as a theory. How can we use it?

A strictly theoretical infallibility is pretty useless, it is just an idea one keeps in one's head until one applies it. When we try to apply papal infallibility and development of doctrine, it seems they cannot go together. To wit, if I cannot know what the pope teaches infallibly, how can I really know what the pope teaches and hence what the truth is? A good example is Unam Sanctam and Lumen Gentium which seem contradictory on their face. I know there are harmonizations online, but I think even if the harmonizations are true there are still a lot of issues.

Unam Sanctam stated that unless one is in fellowship with the pope, one is outside the Church. Lumen Gentium states that e.g. Protestants are separated brethren, with defective fellowship with the pope. Now, assuming that these can be reconciled, I have the following questions:

1) Were all those RC theologians and believers wrong when they took Unam Sanctam as meaning what is says for 700 years, that those not in fellowship with the pope are outside the Church? If they were not wrong, where are the theologians from the middle ages who said those not in fellowship with the pope have a real though defective relationship with him, and were they widely accepted if they did say so?

2) If so many got this infallible document wrong because the pope really mean the e.g. EO are in the Church, but are "true particular churches" with a defective fellowship with the pope, and that the Prots are really "separated brethren", how can we be sure we really understand what the pope's teaching is now, that it will not "develop" into something quite the opposite of what we believe today based on his teaching tomorrow, given what Unam Sanctam states, to wit "herefore, if the Greeks or others should say that they are not confided to Peter and to his successors, they must confess not being the sheep of Christ, since Our Lord says in John 'there is one sheepfold and one shepherd."?

3) If certain parts of Unam Sanctam are infallible while others are not, how do we separate the infallible wheat from the fallible tares?

These are honest questions BTW.

If we try and pick out this or that particular tare, we will raise questions about other tares--whether they are really wheat or not. And if questions are raised more word chopping will occur, until we cannot be sure of what the words mean without an infallible interpreter. And I suppose we have come full circle. In a way, the justification for papal infallibility requires the same concept of an infallible interpreter it attempts to justify. If the words ""Therefore, if the Greeks or others should say that they are not confided to Peter and to his successors, they must confess not being the sheep of Christ, since Our Lord says in John 'there is one sheepfold and one shepherd'" really somehow mean the Greeks have only a defective fellowship with the pope, why couldn't an infallible interpreter simply say that?

I believe this is a real problem with how the RC presents itself as an authority, because ultimately the authority is not clear as to what it teaches, things can develop no matter how clearly worded. Just read Unam Sanctam and see what I mean. Its meaning seems clear, and the Medieval Church seemed to take the view that the pope really did have temporal authority over the princes, the spiritual sword over the profane sword--just ask the German Emperor who's vassals' vows were unilaterally revoked by the pope. If this was not the true Tradition of the RCC, were are the contrary views, and if they exist, why didn't a pope or two say "Yes, that is what we really meant!"?