The Church government is a patriarchy, not a democracy (republic) like the US. There are no checks or balances the way we are used to. A valid pope could not issue an official decree saying that abortion was okay. This is where the faith part comes in. Christ has promised us that it is just not possible. It is possible that the pope could give an opinion or a remark, even a public remark, that abortion was okay, but he’d clearly be wrong and eligible for the millstone award of all time. All the faithful would know he was wrong and after we all stopped foaming at the mouth, we’d kick our prayers into high gear.
It looks like we need to make clear what are matters of doctrine (faith and morals) and what are matters of discipline (laws, customs, and norms.) Doctrine is the teachings of Christ, the same always and everywhere. These are revealed to us in many ways, primarily by Jesus through His Church and Vicar, the pope. This never changes, although at times it is further explained or explained in new ways. Discipline would be the Code of Canon Law, the liturgical rites (how to celebrate mass, etc.), and the hierarchy and government of the Church. These things can change and do all the time. These could and sometimes do change to the detriment of the Church.
You gave the example of a pope rubber stamping abortion and then asked about it becoming legislation. Doctrine is not legislation. If a pope promulgated false doctrine, the faithful bishops would start trying to find out who the real pope is or electing a new. (They wouldn’t be replacing the imposter since he was never pope; they would be electing the successor to the last valid pope.) As far as discipline (legislation) goes, if the pope says it, then it goes. There is no veto, or voting or anything like that. But, the pope does establish committees and get advice and counsel from all types of people at the Vatican and even other places. But he’s never bound to follow what they say. He’s completely the final authority.
You’re perfectly welcome to keep asking questions here and there are many people who will help as best they can. If you want more solid and straightforward answers though try the Catholic Answers tracts. You can find them here:
Church and Papacy.
The Authority of the Pope Part 1
The Authority of the Pope Part 2
I also highy reccomend this book by Patrick Madrid. It should answer a lot of the questions you are asking here in a very clear and reasonable way. Click on the book for more information.
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Pax et bonum