In practice, a dissenting Catholic is actually a Protestant.
No. That is patently false, and not supported by any teaching of the Church.
Would your logic not result in me automatically being able to be the godparent?
No, of course not. No Catholic is “automatically” qualified to be a godparent. The requirements are:
“Only one godparent is necessary. The godparent must be a fully initiated Catholic
(Baptized, Confirmed, received Eucharist), at least 16 years old, and must be leading a
sacramental life in harmony with the church. (Cf. CIC, can. 874) The godparent should be
willing to accept the responsibility of assisting the parents in developing the faith life of
the child.”
As for your brother’s case, an error was indeed made if he did not conform to those specifications. But whether he did or not is not for you and me to decide. In any case, that does not affect the validity of the sacrament, though.
Furthermore, you or I would not have any standing to challenge the priests decision. That would be a matter for a court of canon law.