In what way can the Church re-examine and dismiss, reject or deny (or even truly modify) what it has otherwise taught for centuries without the whole house of cards crashing down?
This is the dilemma of "tradition". Anything you add to the house of cards cannot be easily, if at all, removed, modified, etc, without the whole thing crashing down. If I am wrong, explain to me how I am wrong.
Catholics are required to believe dogma, in order to be Catholic. Nothing else.
Strip everything away from the Catholic Church, leave only the mass, and you still have the Catholic Church.
Once something has been defined, you can't not accede to it and still be a Catholic in good standing. But many things have not been defined, and many may never be. To have trouble accepting a particular teaching is not heresy -- to insist (especially publicly) that you're right over the Church is.
The term "mortal sin" is tossed around pretty carelessly, even by some Catholics. To be mortal sin (the "sin unto death"), the matter involved must be serious ("grievous" is the preferred expression), the person must have full knowledge of its gravity, and the person must give full consent of his will.
The Church doesn't presume to send people to Hell. Some phrase it that not even God send people to Hell -- they send themselves.