Does denying the Catechism leave one a Catholic? Accepting that Jesus had brothers and sisters born of Mary is opposite the Catechism.
If a Catholic were to look at Gen. 8:1 and believe that it assures him that God remembers him and his whole family in the midst of a great trial, he is not prohibited to interpret and have faith that it is so, even though no theologian may have interpreted it precisely that way.
On the other hand, Catholics, like Protestants may run into some difficulties with the authorities in their church if they proclaim an interpretation of scripture that is counter to accepted doctrine. In either case, it is likely that they will be told to hush up and repent. And again, in either case, excommunication would not be the first response to error.