Well, there's absolutely NO "guarantee to receive the same benefits as members". That's clearly overstating the case. What's true is that the Church has left it open that they MIGHT. When the Church baptizes catechumens in imminent danger of death, the Church CLEARLY understands that there's NO GUARANTEE of a BoD. I addressed the Canon Law earlier, with the Church leaving open the POSSIBILITY of their salvation. Prior Church discipline was the OPPOSITE. When the Church requires lengthy instruction, it's because the Church understands that God's providence will keep those destined for Baptism alive until they receive it ... unless He doesn't will that they receive it. So I don't see the problem you're trying to present.
So if catechumens are guaranteed the same benefits as members, why can't they receive the other Sacraments before Baptism?
To answer your final question, because they have to receive Baptism before they can receive any of the other sacraments. Having a guarantee of some future benefit doesn't mean you have already received it.
So if there is no guarantee then it makes perfect sense for a catechumen to insist on being baptised immediately. In fact it would be unjust for a priest/minister to refuse it. An unjust law is no law at all. And since it is legal to baptise a catechumen who is in imminent danger of death then it is not intrinsically evil to baptise a catechumen who hasn't received instruction. But the Church doesn't do that. The rule is that they must receive instruction first.
Also, I don't want to defend +Lefebvre's idea of BOD because I don't agree with it. However, the story about him telling a catechumen not to worry about not having baptism doesn't prove anything. I've never heard of anyone settling for BOD. The most popular definitions of BOD say that it doesn't impart the character of the sacrament. Who would want that? Who upon hearing about BOD would cease to desire the Sacrament of Baptism? Who would forgo the other sacraments for a lifetime because they felt secure that BOD would suffice? There must have been millions of catechumens who had the same thought as the African guy in the +Lefebvre story. Why haven't we heard more about catechumens getting baptised immediately? It must be because they were reassured that they had nothing to worry about as long as they did penance for their sins and obeyed the commandments and precepts and laws of the Church. But they should worry if there is no guarantee of the reception of the sacrament. In fact, I think it would be negligent not to baptise them immediately if death would result in them being damned.
St. Ambrose sums it up pretty well for me. "Undoubtedly because he asked for it he received it." But he asked for the Sacrament, so therefore he must have received the Sacrament at some point. God knows. Maybe he was not sincere and he didn't receive it. But if he was sincere...ask, and it shall be given you: seek, and you shall find: knock, and it shall be opened to you. (Matthew 7:7)
I see what you are saying about the molinism vs. Thomistic doctrine of grace. If the best theologians can't figure it out then I have no chance. And I'm sure that's why a lot of post-V2 traditionalists are perfectly satisfied with repeating what the Baltimore Catechism, Pius X Catechism, Van Noort, Spirago, Ott, Fenton, Suprema Haec Sacra, etc say. Very few people have the desire or resources to solve pre-V2 theological controversies. Most people are content to draw the line at the sacraments. Which makes sense since that is the most visible manifestation of the problem. But there definitely were pre-V2 doctrinal issues.