About your theory of only Catholics in heaven, I thought of another wrinkle this morning.
Why does the Catholic Church then accept the Baptism of individuals from a Protestant Church when they wish to convert to Catholicism?
If those Baptism in other churches are valid?? Hmmmm. Leaving it there for the discussion.
It's not a wrinkle at all; in fact, the Council of Trent states the very fact you speak of:
Session 7, Canon 4: If anyone says that the baptism which is given by heretics in the name of the Father, and of the Son, and of the Holy Ghost, with the intention of doing what the Church does, is not true baptism, let him be anathema.
You're right, they are valid. The reason is because they weren't Protestant baptisms to begin with... there's no such thing as a distinction between a Catholic and a Protestant baptism, IF the baptism is performed properly, with water and in the name of the Father, Son and Holy Ghost, with the intention of doing what the Church does. If it is performed properly, then it is, in reality, a Catholic baptism, whether anyone thinks it is or not.
All properly performed baptisms are Catholic baptisms no matter who they are performed by or where they are performed.
So you're right about that fact, no doubt. But remember, besides being baptised, it is also necessary to hold the Catholic Faith completely, and to keep it "whole and undefiled", as the Church says.
In other words, while you are very correct on the item that properly performed baptisms in protestant churches are in fact valid Catholic baptisms, that fact does not mean that those of who are so baptised have thereafter remained in the bosom and unity of the Catholic Church if they deny any part of the Catholic Faith itself.