It is obvious to me that you do NOT well understand why the Church teaches and why it does as it does.
Priests DO have the authority from Jesus Himself to forgive sins. Like all else with The Church that is based on Holy Scripture.
Your mother COULD have remarried in The Church IF she had had her former marriage dissolved via an annulment.
I know because I've seen it done.
A marriage CAN take place outside the Church building.
My sister-in-law was married on a ranch by a Catholic Priest.
Permission to do that was obtained from the Bishop.
There are several very good reasons what a child must be prepared to receive Christ in the Eucharist.
One is that a person MUST be worthy to partake.
There is much more about all this regarding your post.
I suggest that you contact a good Catholic apologist who has the time to help you fully understand you CHRISTIAN CATHOLIC FAITH.
You are like many who leave The Church, they don't understand it and that all of Catholic teaching IS biblically based.
I'm a convert and needed to fully understand Catholicism before I would join it. That is why I say that anyone who fully understands The Catholic Church will never leave it.
Peace and kindness,