No.
That is not what the law says. A qualified Catholic can be a sponsor. An unqualified Catholic can attend the baptism. He/she cannot act as a non-Catholic witness because he/she is not a non-Catholic.
That was not the question asked. As many people as the priest allows can stand near the baptismal font during the baptism. I’ve seen entire families, 10 or so people.
You quoted the law yourself. It says by exclusion, that a Catholic cannot be a witness. Instead, a “witness” to a baptism, has to be a non-Catholic from an ecclesial community. That is how law works, if a law specifies the who, you cannot substitute someone else. This cannon, by using the phrase “baptized person who belongs to a non-Catholic ecclesial community,” specifically defined the type of person who can be a witness at a baptism. No one else qualifies, and by definition a person baptized into the Catholic faith, is not a non-Catholic.
So you are saying that because a baptised but not yet confirmed Catholic is not specifically mentioned then it means they are excluded.
That is how law works.
If the law specifically defines the person who can fulfill a role you cannot substitute someone else. Otherwise there would be some very large problems, other places where specific people are defined for specific roles: like in cannon law where it says that a validly ordained priest can confect the Eucharist (CCC #900), you cannot substitute just anyone. Or who can received the Eucharist (CCC #912) only baptized persons not prohibited by law.