it's perfectly in order to say "those ideas are certainly Christian ideas, but they are not the only Christian ideas and most Christians disagree with them".

I think CCC is trying to say that those aren't Christian ideas at all and that people who think that that's what Christianity is like are mistaken, not just choosing a smaller group of Christians over a larger one.

It sounds as if you're assuming that improved understanding of Christianity always means discovering more things you're supposed to do. But it could go the other way too

It isn't "you do the exact set of things described by your mistaken understanding of Christianity, and you are saved". It's "imperfect understanding is an excuse for failing to meet the requirement". Improved understanding can only increase the things you must do, never reduce it. In other words, if you falsely think that Christianity requires being a vegetarian, and you fail to be a vegetarian (thus violating your mistaken understanding of it, but not actually violating true Christianity), you can still be saved.

But that's not the same as being able to think of plenty of things Christianity says you have to do, on pain of damnation.

Everything that Christianity says you should do, is under pain of damnation (or has no penalty at all). It's not as if God has some other punishment short of damnation that he administers instead when your sin is mild.