You are using a totally fake catechism, Santo Subito.
I sometimes read Catholic Answers, and if it weren't so sad, it would be hilarious to watch these people try to figure out tricky moral questions. All morality in that imitation of the Church is random and manmade, just like it is with the Prots, because the Holy Ghost is not guiding the "Pope" ( who is not Pope, so that's to be expected ).
One thread was about how illegal immigration was a mortal sin. Meanwhile, others defend it vociferously, as so many in the Novus Ordo seem to do. Those who said it was a "mortal sin" refer to how illegal immigrants don't pay taxes, while in the CCC it says that this is theft and against the Seventh Commandment.
Okay, first of all, cheating on taxes is not theft and was never considered such before Vatican II. We may have a moral obligation to pay taxes, but that is not the same as theft, because the money doesn't belong to the government; it's ours and they simply take it for various civic purposes, hopefully ( and sometimes evil purposes ). And if a Mexican immigrant who picks lettuce doesn't pay his taxes, the government is losing such an infinitesimal amount compared to the trillions they have... And a lot of it is funny money anyway. So at worst, it would be a venial sin, most likely.
So right here, flowing from the error that cheating on taxes is the mortal sin of theft, you get the ridiculous Pharisee notion that illegal immigration is mortal sin. By this logic, the Catholics who infiltrated England to bring the Mass to the Catholics under Henry VIII were mortally sinning.
Then you have others who see no problem with just breaking every civic law, like the bishops who encourage illegal immigration in VII.
I don't know the answer, but I do know that these extremes these people go to are absurd. Illegal immigration is a big grey area. We should obey just laws, but how just many of them are is in debate. Also, the law against illegal immigation is basically on paper only. Meaning it's not really enforced, and is even secretly or not-so-secretly encouraged by this blatant laxism. It's like the speeding laws. Does anyone really go 60 on the freeway? You'd probably be ticketed if you went that slow in the fast lane! So it is not a mortal sin, but clearly some of these immigrants should try harder to make things work in their own nation.
Vatican II is a total grab-bag, it has nothing to do with real morals. You are poisoning your brain reading that junk.