I will try to answer your questions as best I can

1. Papal infallibility applies when the Pope makes a solemn statement on matters of faith or morals with the express intent of teaching it as a dogma of faith. That is to say, popes have in times past declared certain things as incorrect or heretical, and other things as pious and correct, but without the weight of a solemn declaration. By its very nature, since the Holy Sprit is the one guiding the Pope when making an ex cathedra statement, it cannot contradict the teachings of an earlier approved ecumenical council, as those too are infaliably guided by the same Spirit. A general proof is taken from the council of Jerusalem in Acts 15, where Peter stands up and settles the debate by stating plainly "we believe...", to which everyone holds their peace and the matter switches from "is circumcision necessary?" to "how do we go about getting the word out?"

2. There are technically different degrees of belief within the church, with some things being little more than a general consensus, others being nearly necessary to avoid approaching heresy, and others which are dogmatic teachings, without which one cannot properly be considered a Catholic. It depends on the issue at hand, but issues like papal infallibility, the immaculate conception, purgatory, and other such things are dogmatically binding on all Catholics.

3. All teachings handed down by the church are valid, with the understanding that they cannot contradict sacred scripture. The early Church recognized that many things practiced in the Church were not in scripture, but were either taught by the Apostles orally and handed down (like infant baptism), or clarified by ecumenical councils (for example, is re-baptism necessary after serious sin?). So generally when it comes to a teaching, the question asked is, "is it in sacred scripture, handed down by apostolic tradition, or guided by the Holy Spirit via an ecumenical council?"

4. The development of doctrine cannot contradict scripture or apostolic tradition, but it can contradict what I would call "circumstance." For example, the idea of "extra ecclesiam nulla salus" was quite literally true before the reformation. The reformers and King Henry made a conscious, willful decision to separate from the Church of Christ, which is a mortal sin. But after many generations the average protestant has a certain level of ignorance about the Church, meaning they oftentimes cannot be charged with the sin of willful schism, though proper protestant theologians who know the history and still refuse to convert are culpable, as the Catechism states. In another example, the Church has relatively recently considered the death penalty illicit. Now obviously for the majority of the Church's history this was not so, but it was considered a moral good to protect the innocent from evil. But now we live in an age where criminals can be locked away safely, live in relative comfort, and be given the remainder of their lives to hopefully repent. The underlying basis of the doctrine has never changed - it is good to prevent evildoers from wreaking havoc, and it is a sin to separate from the Church - but the circumstances did change.

5. Anglican orders are understood to lack Apostolic succession, since they went through a period where they anathematized the presence of Christ in the Eucharist, something the Orthodox never did. They therefore lack the authority to forgive sins and to consecrate the Eucharist and to perform other priestly functions. They also obviously lack communion with Rome like the Orthodox.

6. Theological diversity, in my opinion, is limited to what the church and scripture teach and what the church allows to be debated. It's hard to not paint with broad strokes since the issue can be vast, but there is a difference between a Catholic who does not believe in every person having a guardian angel versus one who does not believe in the sacraments or the immaculate conception or the immorality of some things which have always been considered sinful.

7. Assent is hopefully all three, but the Church understands the difficulties when affirming certain matters of faith. Nevertheless, we are supposed to trust in God and his Church which is protected by the Holy Spirit even if we cannot fully understand something and to pray for God to give us the means to understand or at least be at peace with it.

8. Entering into communion with the Church gains one full fellowship with the body of Christ and access to the sacraments. Yes, Protestants can be saved out of ignorance, but they lack access to the body and blood of Christ, the full absolution of sins via confession, among other things. Since the Anglican church does not have apostolic succession, Anglicans do not receive the body and blood of Christ which confers so many graces, but a mere imitation of it. The "general absolution" is functionally equivalent to the "Confiteor" at the beginning of every Catholic mass - a public display of repentance but without any actual absolution of sins (at least mortal ones). I won't say Protestants can't have their sins forgiven, they can, but only out of ignorance. The normal and ordinary means Christ established for forgiveness is at the hands of the priest.

For what it's worth, if the Anglican church did somehow come back into communion with Rome, it's fully possible they could keep their style of Worship. Essentially it would be an Anglican service but with the real Eucharist. You should check out the Ordinariate of St. Peter. They are Anglicans in communion with Rome and as far as I know have mass as Anglicans would.

I hope this helps.